Added gunzip() wrapper to utils.system_tools
On-the-fly decompression of fits.Z
"Gracefully" fail on gzip unavailable
Improved gunzip() when gzip not available
gunzip() already returns the decompress file name
Also look for *.fits.Z in cache
Merge pull request #305 from jwoillez/eso_gunzip

On-the-fly decompression of downloaded fits.Z files