compress temporary *.npy files with gzip to give *.npy.gz files

[JamiePringle]

This pull request compresses the .npy temporary files with the gzip module. There should be no user facing changes, unless they monitor the out- directory to see how far along things have gotten. In that case they need to use gzip.open() to open the *.npy.gz files.

Any code that processes the temporary files in the out-* directories will need to be modified, following the pattern in this pull request.