title: Set array to NAN if less than x amount of data points are in a datetime
body: <p>I have a dataframe output like the following. </p>
  
  <pre><code>dateparse = lambda x: dt.datetime.strptime(x, "%d:%m:%Y %H:%M:%S")
  df = pd.read_csv('somefile', skiprows = 6, na_values = ['-999.000000'], parse_dates={'times':[0,1]}, date_parser=dateparse)
  df = df.set_index('times')
  print(df)
  
                      times           A          B          C          D  \
  0     2003-01-01 00:34:38    0.082516   0.102096   0.143908   0.227867   
  1     2003-01-01 00:38:57    0.085833   0.106582   0.149704   0.238216   
  2     2003-01-01 00:44:07    0.084019   0.103900   0.146425   0.232553   
  3     2003-01-01 00:50:18    0.085005   0.105516   0.148011   0.235786   
  4     2003-01-01 00:57:57    0.089478   0.110041   0.155636   0.247485   
  5     2003-01-01 01:07:31    0.090858   0.112727   0.157333   0.250582   
  6     2003-01-01 01:18:11    0.095183   0.118650   0.166181   0.266422   
  7     2003-01-01 01:24:54    0.095459   0.118206   0.165703   0.265689   
  8     2003-01-01 01:33:24    0.096138   0.118215   0.165815   0.264526   
  9     2003-01-01 01:48:11    0.093621   0.116163   0.162775   0.260970   
  10    2003-01-01 02:03:11    0.097354   0.120059   0.167964   0.270480   
  11    2003-01-01 02:18:12    0.095817   0.118176   0.166490   0.266451   
  12    2003-01-01 02:24:50    0.094345   0.113859   0.161243   0.256563   
  13    2003-01-01 02:33:12    0.093151   0.112366   0.158082   0.252427   
  14    2003-01-01 02:48:12    0.088426   0.108625   0.152767   0.246545   
  15    2003-01-01 03:03:11    0.090825   0.111603   0.156169   0.252650   
  16    2003-01-01 03:18:12    0.088676   0.107009   0.150868   0.241441   
  17    2003-01-01 03:24:51    0.091120   0.107680   0.153139   0.245209   
  18    2003-01-01 03:33:12    0.094014   0.113787   0.159610   0.256748   
  19    2003-01-01 03:48:13    0.090774   0.110138   0.155652   0.252272   
  20    2003-01-01 04:03:12    0.091755   0.111134   0.158139   0.256281   
  21    2003-01-01 04:18:12    0.105797   0.127926   0.182425   0.300056   
  22    2003-01-01 04:33:12    0.108270   0.130971   0.184997   0.309122   
  23    2003-01-01 04:48:13    0.112102   0.136731   0.192955   0.323832   
  24    2003-01-01 05:03:12    0.113862   0.139115   0.195716   0.330440   
  25    2003-01-01 05:18:13    0.109960   0.134348   0.187129   0.317269   
  26    2003-01-01 05:24:50    0.111670   0.133918   0.187268   0.315839   
  27    2003-01-01 05:33:12    0.115415   0.139826   0.195095   0.330612   
  28    2003-01-01 05:48:11    0.108153   0.130107   0.181749   0.311513   
  29    2003-01-01 06:03:11    0.124846   0.150228   0.209006   0.360110   
  ...                   ...         ...        ...        ...        ...   
  62094 2012-12-30 02:32:45    0.076481   0.082212   0.098530   0.127626   
  62095 2012-12-30 04:02:46    0.074581   0.075652   0.093802   0.121700   
  62096 2012-12-30 04:17:45    0.069553   0.069642   0.087689   0.111479   
  62097 2012-12-30 04:24:45    0.078354   0.078970   0.098119   0.129527   
  62098 2012-12-30 04:32:46    0.082699   0.081013   0.102228   0.134887   
  62099 2012-12-30 04:47:45    0.085212   0.089094   0.108791   0.141562   
  62100 2012-12-30 07:56:36    0.085549   0.090656   0.110367   0.158078   
  62101 2012-12-30 08:01:01    0.088070   0.094420   0.114416   0.164917   
  62102 2012-12-31 01:35:43    0.199741   0.223171   0.286124   0.447926   
  62103 2012-12-31 01:48:13    0.217590   0.244189   0.308924   0.477287   
  62104 2012-12-31 02:18:12    0.209208   0.229590   0.283930   0.422209   
  62105 2012-12-31 02:25:06    0.225229   0.245545   0.301449   0.445500   
  62106 2012-12-31 02:33:12    0.236910   0.258252   0.313862   0.460853   
  62107 2012-12-31 03:03:13    0.228002   0.246693   0.302313   0.445168   
  62108 2012-12-31 03:18:13    0.235366   0.257824   0.317586   0.475176   
  62109 2012-12-31 03:25:07    0.246707   0.269936   0.332616   0.498684   
  62110 2012-12-31 03:33:13    0.246553   0.269369   0.330051   0.490452   
  62111 2012-12-31 04:25:10    0.249062   0.268720   0.320451   0.454450   
  62112 2012-12-31 04:33:13    0.251103   0.269946   0.321919   0.456619   
  62113 2012-12-31 04:48:13    0.257122   0.278713   0.335067   0.488326   
  62114 2012-12-31 05:03:14    0.274840   0.298629   0.361423   0.536313   
  62115 2012-12-31 05:18:14    0.272331   0.297988   0.361081   0.540261   
  62116 2012-12-31 05:33:13    0.259284   0.280575   0.336578   0.490123   
  62117 2012-12-31 05:48:13    0.281216   0.305000   0.364803   0.533686   
  62118 2012-12-31 07:03:13    0.316535   0.346806   0.418629   0.643321   
  62119 2012-12-31 07:18:13    0.287614   0.315958   0.383844   0.597279   
  62120 2012-12-31 07:46:05    0.323202   0.350592   0.430860   0.657584   
  62121 2012-12-31 07:52:16    0.288645   0.319988   0.389610   0.622262   
  62122 2012-12-31 07:57:24    0.294492   0.328670   0.398978   0.638286   
  62123 2012-12-31 08:01:45    0.287507   0.320027   0.391866   0.636343  
  </code></pre>
  
  <p>What I'm hoping to achieve would be saying if any of the index 'times' has less than 30 points for that year-month-day (ie less than 30 datapoints for 2003-01-01 or any other date), set A,B,C,D = NAN. Is there an easy way to do this?</p>