[3pt] Consolidate bash environment variables into a single file

[mluck]

Consolidate bash environment variables into a single file.

Current behavior

Some environment variables are defined in each bash script, therefore they may be duplicated which makes them unnecessarily difficult to manage. For example, export extent=GMS exists in 3 files: gms_run_unit.sh, gms_run_branch.sh, and gms_run_post_processing.sh.

We also want to be able to use an environment variable to set the target projection (e.g., EPSG:5070) in gms_run_post_processing.sh so this will add on to PR #782.