accordingly to the scope of the class which now is not a specific
contrast but a way to represent contrast. This is more consistent with
what the data represents and how to work with the data.

only two for testing reason
subclassing a subcalls of numpy was hard due to the use of __new__ and
__array_finalize__
max recursion errors appear now after multiple use of derive. Do I need
numpy's __del__ method?

just as a reference implementation for the value contrast and not generacilly

reference implementation for Movie, Frames class
scatterplot contrast without plot as function in contrasts module
helper functions in helpers module