Move all tutorial code snippets to tests

[mattldawson]

Some tutorial sections have embedded code snippets. These should be moved to tests so the code doesn't become stale.

Acceptance Criteria

• No .rst files in docs/source/ contain source code

[K20shores]

The only code I can find is in https://ncar.github.io/micm/user_guide/user_defined_rate_constant_tutorial.html, but that's not code, it's a diff which we can't show without putting code into the tutorial. I vote we close this @mattldawson