Re-organize tests

[mrp089]

Current situation

Re-organized test cases as mentioned in #95 by physics:

• separate test_*.py files
• subfolders

Also, I simplified the test case definition:

• removed redundant variables by using default names for input file, result file, and number of time steps
• move test functions to conftest.py
• move number of processors to conftest.py
• use default tolerance of 1e-12

Release Notes

Should simplify adding new tests.

Documentation

We still need to add a documentation and testing guide.

Testing

Runs the same 75 tests as before that pass locally on Apple M2 (except cmm, as expected). I will verify that the coverage is the same once the GitHub actions are done.

Code of Conduct & Contributing Guidelines

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