Compresses/uncompresses a set of files using gzip compression.
  When compressing (uncompressing), each of the GenericDataFile
  of the file set are compressed (uncompressed).
# S3 method for GenericDataFileSet
gzip(this, ...)
 # S3 method for GenericDataFileSet
gunzip(this, ...)Returns (invisibly) itself.
Arguments passed to gzip()/gunzip() on each
   of the GenericDataFile entries.
Henrik Bengtsson
Internally gzip and gunzip are used.
  For more information see GenericDataFileSet.