Do not store the MigrationMatrix using unique_ptr in DiscreteDemography.

[molpopgen]

This design decision was convenient when DiscreteDemography was a standalone C++ class. (The unique pointer would store nullptr for "no migration", returning None to Python.) However, it leads to complications implementing deepcopy for the demographic model.

A likely side-effect is that accessing a model's MigrationMatrix may trigger a copy in the not-None case.

Solution: store an object, not a pointer. The object can easily model the concept of "empty migration matrix" without relying on nullptr. Adding an empty() member function would suffice. Then, the Python side would return None if empty, or a reference to the object otherwise.