Name and organisation of tests

[lionelkusch]

The number of tests is increasing to avoid being lost in the testing some parts of the code. I think it's important to choose a good organisation of tests and correct names.

I propose to follow the name suggestion of numpy:
https://numpy.org/doc/stable/reference/testing.html#writing-your-own-tests

The general idea is to create a file test_....py for each python file. Each test has a name starting with 'test_'. If an aspect requires multiple tests to be tested, these tests are grouped into a class.

In addition to it, we need to one-line docstring to limit the tests to a specific aspect and to document the tests.