Two validity check functions for collisions and self-collisions

[lucarossini-iit]

When initializing the validity checker functions, there are two equal functions that checks the collisions/self-collisions. One is in validity_checker_context.cpp (make_collision_checker()) and one is in validity_checker_factory.cpp. The first one is used. Is there a reason why two functions were written? @alaurenzi @EnricoMingo

[EnricoMingo]

I think one of the two is useless, is it possible to understand which one are we using currently in the code?