Ownership semantics for the marginal trees.

[molpopgen]

We currently store an object and return a const reference via a getter. This design is rather inelegant. We should refactor to use shared pointers. The getter returns a pointer to const object to prevent modification by the caller.

[molpopgen]

Getting this right is a bit annoying. On the caller's side, one must use auto && to properly receive the shared_ptr as const.

[molpopgen]

This should be handled along with #294, at which point we should deprecate the existing types that do the const & method.

[molpopgen]

Random thought: this may all be best handled by a get method returning const *?

[molpopgen]

Random thought: this may all be best handled by a get method returning const *?

No -- you can always delete a pointer. Seems like the smartest thing here is the shared/weak pointer duo.