While there is still a lot of work to do (mainly in Registry, Quantity, Unit),
this large PR makes several changes all around the code. There has not been
any intended functional change, but certain typing improvements required
code minor code refactoring to streamline input and output types of functions.

An important experimental idea is the PintScalar and PintArray protocols,
and Magnitude type. This is to overcome the lack of a proper numerical
hierarchy in Python.

Note: the word 'plain' was chosen instead of 'base' as BaseUnit or BaseQuantity
as the latter might be interpreted physically and not a part of the class hierarchy
as intented.