title: Using rollmean when there are missing values (NA)
body: <p>I have a data set which has a couple of <code>NA</code> in it. I take a rolling mean and expect that when there is no <code>NA</code> in the window, the rolling mean should produce a number as opposed to <code>NA</code>, however, <code>rollmeanr</code> in <code>zoo</code> does not seem to do this. Example:</p>
  
  <pre><code>require(zoo)
  z = zoo(cbind(a=0:10, b=c(NA,10:1), c=sample(1:11,11)), 1:11) 
  rollmeanr(z, k=3, fill=NA)
      a  b        c
  1  NA NA       NA
  2  NA NA       NA
  3   1 NA 3.333333
  4   2 NA 4.666667
  5   3 NA 4.000000
  6   4 NA 6.333333
  7   5 NA 7.000000
  8   6 NA 9.333333
  9   7 NA 8.333333
  10  8 NA 8.666667
  11  9 NA 5.666667
  
  rollapply(z, width=3, FUN=mean, by=1, by.column=TRUE, fill=NA, align="right")
      a  b        c
  1  NA NA       NA
  2  NA NA       NA
  3   1 NA 3.333333
  4   2  9 4.666667
  5   3  8 4.000000
  6   4  7 6.333333
  7   5  6 7.000000
  8   6  5 9.333333
  9   7  4 8.333333
  10  8  3 8.666667
  11  9  2 5.666667
  </code></pre>
  
  <p>I would expect these two calls to generate the same result. Please comment. Some session info:</p>
  
  <pre><code>sessionInfo()
  R version 3.0.1 (2013-05-16)
  Platform: x86_64-unknown-linux-gnu (64-bit)
  
  locale:
   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
   [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
   [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
   [7] LC_PAPER=C                 LC_NAME=C                 
   [9] LC_ADDRESS=C               LC_TELEPHONE=C            
  [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
  
  attached base packages:
   [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
   [1] zoo_1.7-10
  
  loaded via a namespace (and not attached):
   [1] grid_3.0.1      lattice_0.20-15
  </code></pre>