give memDecompress a compression type argument

* and pass on ...
fix reading of gzipped nrrds on windows

* somewhat simplifying the read.nrrd code in the process
* strange thing is I had a note that the approach (using a gzcon to wrap an
  open connection) failed with earlier versions of R.
Merge branch 'fix/read.nrrd-gzip-windows'

* the first change turns out not to be required for this fix but is a small
  improvement that might be helpful in other circumstances.
* fix/read.nrrd-gzip-windows:
  fix reading of gzipped nrrds on windows
  give read.zlib a compression type argument