Create a memoization decorator for relevant calls

[pshriwise]

Description

The Geometry, Cell, Universe, and Material (maybe others too) use a memoization to accelerate searches for contained cells, universe, materials, etc. As a result, there is a lot of repeated code related to managing the memo object throughout. I believe a decorator for the relevant methods could be used to reduce the boilerplate involved w/ these calls.

Alternatives

Leave it alone

Compatibility

No compatibility issues w/ this change.