Change filenames

[M-Lampert]

Currently files containing a class are named in CamelCase. We should follow PEP8 convention and name them in lowercase because this can otherwise lead to confusion during imports. Example: You would think that by importing the following, you would have the Graph class.

from pathpyG.core import Graph

Instead you now have the module Graph and would need to instantiate the class Graph as follows:

g = Graph.Graph(...)

or

from pathpyG.core.Graph import Graph

I think we should rename all files containing classes and import the classes in the modules __init__.py so that we could import e.g. Graph in all of the following ways:

from pathpyG import Graph
from pathpyG.core import Graph
from pathpyG.core.graph import Graph

I hope this will not lead to any circular imports! XD