'GdxCompress' setting is ineffective on case-sensitive OS (macOS, Linux)

[Tyler-lc]

What is this about?

### Issue
On case-sensitive file system (MacOS and Linux), GAMS is not compressing the output files as expected. This results in output gdx files that can be 3-4 times larger than on machines running Windows (which does not have a case sensitive file system).

Cause

The problem seems to be in a capitalization in the file "auxiliary_settings.gms". Currently we have:
"$SETENV GdxCompress 1"

However, it should be in all-caps:
"$SETENV GDXCOMPRESS 1"
The typo causes the command to be ignored on MacOS and Linux.

Proposed fix

in the file "auxiliary_settings.gms" change the relevant line from:
"$SETENV GdxCompress 1"
to
"$SETENV GDXCOMPRESS 1"

[glatterf42]

Closing this since #965 was merged :)