Add a version check to load script

[xylar]

If the load script is for a different polaris version, a developer is told to rerun the configure script.

Checklist

• [x] Developer's Guide has been updated
• [x] Documentation has been built locally and changes look as expected
• [x] Testing comment in the PR documents testing used to verify the changes

closes #155 closes #154

[xylar]

Testing

If the version in polaris/version.py matches the version in the load script, the everything proceeds as usual when the load script gets sourced. If the versions don't match, the output is something like:

$ source load_polaris_test_morpheus_gnu_openmpi.sh 
This load script is for a different version of polaris:
__version__ = '0.2.0'

Your code is version:
__version__ = '0.3.0-alpha.1'

You need to run ./configure_polaris_envs.py to update your conda 
environment and load script.

[xylar]

@sbrus89, this is ready for your review. Hopefully, it's pretty quick and easy.

[xylar]

@sbrus89, a reminder that this is waiting for your review (presumably by inspection).

[xylar]

Thanks @sbrus89!