Beautify + improve code in general.

[mcw92]

• Replace unnecessary list comprehensions with generators.
• Consistently use numpy arrays / lists or rather just request an "indexable" object in the user interface via Union[list, np.array, python array, xarray]). Create new type Indexable with typing.
• Defaults of integers should not be None; rather -1 or 0.
• In __eq__(): If I check for equality of different types, just return False and do not use assert(). if not isinstance: return False