I added a lot of test to test the missing things in the fluids and solids class.
In addition, I changed the Dunder
__repr__ such that is just invoked if really that class is called; and not through a
super().__init__(...) of a child class.
@fjonas: I also changed sometimes the init methods of the childs. E.g.
gorkov/materials/fluids.py Line 26 and
gorkov/materials/solids.py Line 24