Add py-tools variant and set $ISSM_DIR Environment Variable

[lawrenceabird]

This PR adds the following functionality:

• New py-tools variant to install Python function files to <prexix>/python-tools/. These files are necessary to A) access the wrappers that are included in <prefix>/lib/ and B) to conduct various pre- and post-processing of ISSM models. Note: Python files are stored in various subdirectories (e.g. src/m/mesh, src/m/mask, src/m/solve) in the ISSM directory. Here, we flatten this structure to just save all Python files in one directory (<prefix>/python-tools/) as the subdirectory structure serves no functional purpose.

• Export $ISSM_DIR environment variable. This is required by various Python wrappers to obtain information about the run environment.

This PR addresses Issue #333.

Functionality was tested in ACCESS-NRI/ACCESS-ISSM#25

[harshula]

Hi @lawrenceabird , I find this Spack API reference page useful: https://spack.readthedocs.io/en/latest/package_api.html
e.g. https://spack.readthedocs.io/en/latest/package_api.html#spack.package.find

[lawrenceabird]

Completed requested updates. Confirmed successful redeploy here and confirmed functionality by testing the WIP MISMIP example here.

This PR is now ready for re-review @aidanheerdegen and @harshula.

[lawrenceabird]

This new functionality was used in this PR to test a new deployment of an updated ISSM code base on Gadi. This deployed successfully.

[aidanheerdegen]

Looks good.

[lawrenceabird]

@harshula and @aidanheerdegen -- I've addressed all comments and confirmed that these changes still work (here). I subsequently added an access-release version in preparation for a new "release" branch in our ISSM forked repo. This untested addition should have no impact on this PR.

[aidanheerdegen]

LGTM.

If you're happy to do so we generally do a squash merge unless there is a reason to keep separate commits, in which case a rebase to clean-up the commit history would be the way to go.