title: Proper way to subset big.matrix
body: <p>I would like to know if there is a 'proper' way to subset big.matrix objects in R.  It is simple to subset a matrix but the class always reverts to 'matrix'.  This isn't a problem when working with small datasets like this but with massive datasets but with extremely large datasets the subset could still benefit from the 'big.matrix' class.</p>
  
  <pre><code>require(bigmemory)
  data(iris)
  # I realize the warning about factors but not important for this example
  big &lt;- as.big.matrix(iris)
  
  class(big)
  [1] "big.matrix"
  attr(,"package")
  [1] "bigmemory"
  
  class(big[,c("Sepal.Length", "Sepal.Width")])
  [1] "matrix"
  
  class(big[,1:2])
  [1] "matrix"
  </code></pre>