Add convergence tasks for space and time only

[cbegeman]

This PR extends the convergence test framework to accommodate convergence in space, time or both. This PR also adds additional tests for space-only and time-only convergence for the cosine_bell, geostrophic, manufactured_solution, and inertial_gravity_wave groups. The convergence_time cases have not been optimized (in terms of resolution and time steps) nor have separate convergence thresholds been specified.

Checklist

• [X] User's Guide has been updated
• [X] Developer's Guide has been updated
• [X] API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• [X] Documentation has been built locally and changes look as expected
• [X] Testing comment in the PR documents testing used to verify the changes
• [X] New tests have been added to a test suite

[cbegeman]

Testing

I have tested all of the convergence tests on chrys with intel, impi.

[cbegeman]

@sbrus89 If you want to review, feel free!