Remove test groups

[xylar]

In order to increase the flexibility in Polaris, we have removed test groups from the framework. Tasks can still be organized by a common theme in the spirit of test groups but this is no longer a requirement.

The constructors for test groups have been replaced by simple functions for adding the tasks directly to each component. The subdirectories for the test groups have been prepended to the subdirectories for the tasks instead.

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

Based off of #110, which is based off of #112

[xylar]

Note

I do not plan to clean up the tutorial until after the reorganization (including shared steps and perhaps task-parallelism) is finished. It's too much work and too much of a moving target.

The rest of the docs will also need a careful look. I have attempted to update them but haven't had time to give them a thorough read and that more thorough clean-up will also have to wait.

[xylar]

Thanks for the suggestions. I believe I have things cleaned up but will run the pr suite to make sure things still work.

[xylar]

Testing

I ran the pr suite using a recent main as a baseline. All tests passed.

[xylar]

Thanks once again, @cbegeman!!