fits_open: Wrap filenames in "" before spawning subprocesses, to allow spaces

Minor set of fixes to fits_open.pro: We encountered a bug where filenames that had spaces in their directory paths, and ended in .gz, caused the gzip step to fail. By wrapping the filename in spaces in the spawn command, spaces and other atypical characters such as dollar signs and parenthesis are acceptable in filenames and directory paths of gzipped files. 

Note: I made my fixes based on a copy of fits_open from the web site (http://idlastro.gsfc.nasa.gov/ftp/pro/fits/fits_open.pro) which includes some April 2014 modifications that aren't present in the github version of the library for some reason. Presumably you know what's going on here better than I do. I just mention this because that is the source of most of the diffs in this pull request, and my edits are comparatively small being just the addition of some " characters on the spawn lines.