* Allow Python option filter functions to return None

The option filter universe is modified in place by the filter methods
(strikes, expiration, etc.), whose return value is only necessary for
chaining. A Python filter function that returns None would kill the
algorithm with a NullReferenceException.

* Allow C# option filter functions to return null and add regression algorithms

Adds C#/Python regression algorithms asserting that option filter
functions are allowed to mutate the universe in place and return
null/None instead of the universe, which is only necessary for chaining.