Updates to Gemenviron scripts

[mzuranski]

@mzuranski I've long boggled a better approach for the variables set in these scripts, but haven't settled on a solution. I think we should add code that dynamically sets that variable based on the directory location of the source script, but also add code that detects if that variable is already set and not over-ride it. Thoughts?

Originally posted by @akrherz in #57 (comment)

As a standard install step, users need to update the NAWIPS variable in Gemenviron and/or Gemenviron.profile to match their install directory. The current setting of /home/gempak/GEMPAK7 does not match the directory name that Gempak will be extracted to. Making this and possibly other variables more dynamic would be a nice QOL feature.

[mzuranski]

Another nice QOL feature here would be to improve the detection of the Makeinc file to use for Ubuntu. Can we find a way to get rid of this install step too?

cd ~/NAWIPS/config
mv Makeinc.linux64_gfortran Makeinc.linux64_gfortran.orig
ln -s Makeinc.linux64_gfortran_ubuntu Makeinc.linux64_gfortran