Reorganize parallel module and add PBS support

[xylar]

This will be needed for Aurora support.

This merge reorganizes the polaris.parallel module into classes to avoid a growing chain of if/elif/else statements that were arising.

Checklist

• Developer's Guide has been updated
• API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• Documentation has been built locally and changes look as expected
• Testing comment in the PR documents testing used to verify the changes

[xylar]

I need to test both that this doesn't break things on Chrysalis and that it works on Aurora before we proceed...