Implemented State using ABC base class

This MR adapts the implementation of state to use python's ABC base classes instead of extending dict.
This change allows to iterate over the variables in a module.

print("This module uses", len(state), "variables.")
for key in state:
    print("This module is using the variable", key)

Description

Some dict-operations could not be used on state variables (see #71).
This MR solves this by letting collections.abc.MutableMapping check if any functions are missing for a complete dict-implementation.

Things done in this MR

• derived state class from MutableMapping
• removed base class from temporary_state as it has never been used
• added unit tests to check for modules local dict-operations, like iter(state) and len(state).

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created