Creates tests for distribution procedures

[r-akemii]

Added tests for all distribution procedures (IPF, Cabibrate Gravity, and Apply Gravity).
New testing files were also added to the project.

Still to be done:

• Test IPF from Import from layer
• Parametrize test_apply_gravity to consider the user input of an inverse power and Gamma models. Currently, we test if the model reads a .mod file and if the output is working
• The assertions in test_apply_gravity can be improved
• Test saving the procedure report log (I think that only testing if the saving button of this window is working is enough)