Context managers in MontePy?

[tjlaboss]

I brought up the idea of using a context manager for sub input files in #143.

More broadly, context managers could be used to build problems more lazily. Newly created cells could be automatically assigned parameters (universe, importance, etc.), numbered, and added to the problem with. Parts of a problem input could be formatted differently, as suggested in #103.

[MicahGale]

So almost like a factory context? I tentatively like this idea though it's starting to sound like java.

The key will be to make the signature make sense.