Easily check if a cell is "actually" in a problem, and build a cell tree

[MicahGale]

Is your feature request related to a problem? Please describe.
In MCNP transport is only performed on Universe 0. If cells are in other universes to be included it ultimately needs to be filled into a cell in Universe 0. So checking if a cell will be involved in transport is needed. This can be done manually and to do it well requires recursion. I expect most users will not be good at recursion.

Describe the solution you'd like
To add a property to cells that is like cell.in_transport or cell.in_problem or cell.used. This will check if it is ultimately reachable by universe 0.

This should also show the trees of the universes a cell is in something like:

>>> cell.universes
1 > 3 > 5 > 0

Though this should be not a string.

When crawling the tree caching and validation will be essential due to this being a complex data structure with external mutation.