Use weakref

[MicahGale]

There are a lot of circular pointers in MontePy. Weakrefs should be used in someplaces to avoid these circular pointers, making problems un garbage collectable. Likely self._problem should be a weakref.

This is somewhat related to #45.

[MicahGale]

marked this issue as related to #97

[MicahGale]

marked this issue as related to #98

[MicahGale]

In my naïve tests this might not actually be necessary. I think we need to do a more thorough investigation (hence Summer-sprint) before deciding on a go/no-go.