Releasing ID after creating dummy universe for cell plotting

[pshriwise]

Description

This might be overkill, but while reviewing @shimwell's PR providing the Geometry.plot option I realized that creating a Universe object in the Cell.plot method might reserve a bunch of IDs under the hood and potentially confuse users later on during model generation.

Here I've added a function to the mixin module that allows us to release IDs for a given type. This way the next Universe object created after a Cell.plot call will have the same ID as it would have in the absence of the plot call.

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Good find on this -- suggestion for simplification below: