Support custom PYTHONUSERBASE
The DESC conda environments now support defining a user area to store locally installed packages.
Three new environment variables are now supported:
DESCPYTHONUSERBASE - for use with desc-python
DESCPYTHONBLEEDUSERBASE - for use with desc-python-bleed
DESCSTACKUSERBASE - for use with desc-stack-weekly or desc-stack-weekly-latest
When defined, the set up script for the DESC environment will update PYTHONUSERBASE and PYTHONPATH. When new packages are installed via pip install --user, the package will be installed in PYTHONUSERBASE and found during execution due to the updated PYTHONPATH.