title: Modify NDarray for specific columns
body: <p>I have a 2d NDarray, of shape (12,8), like that:</p>
  <pre><code>[[ 89  65   4  86  44 137 113 124]
   [ 88  71   2  89  40 140 109 129]
   [ 93  71   5  87  40 139 111 129]
   [ 87  74   6  96  47 143 113 129]
   [ 81  74   3  99  47 144 112 129]
   [ 86  64   4  89  47 139 115 123]
   [ 85  76   1  93  38 142 106 132]
   [ 89  80   4  94  38 143 107 134]
   [ 84  68   4  93  48 141 114 125]
   [ 95  42  14  80  65 130 135 107]
   [ 90  35   1  67  50 124 123 104]
   [ 68  36   1  84  63 129 126  97]]
  </code></pre>
  <p>I need to apply a simple function <code>val = 200 - val</code> to all odd-numbered columns. How could I do that?</p>