Add missing unit tests

[adigitoleo]

Tracking issue for missing unit tests. It would be good to have unit tests for this stuff as well:

• Voigt averaging (minerals.voit_averages)
• Reading parameter file (io.parse_config)
• M-index calculation (diagnostics.misorientation_index, there are some tests for orthorhombic lattice only, but they might need improvement also)
• diagnostics.finite_strain
• io.extract_h5part
• stats.resample_orientations
• stats.misorientations_random
• All public methods in velocity