[bugfix] Deepcopy inherited variables from the parent classes

[jjotero]

This PR fixes the upstream propagation of changes to variables done by the child classes. Reproducer:

class Foo(rfm.RegressionMixin):
    v = variable(int, value=3)

class Bar(Foo):
    v = 4

Foo().v # Erroneously printed 4 instead of 3
Bar().v # Prints 4

A unit tests has been extended to catch this.