Update uniform mesh converter to use optimal mesh size when material boundaries cannot be honored

[mgjarrett]

https://github.com/terrapower/armi/blob/cfc23356334253cb714171349f51acceb2db0225/armi/reactor/converters/uniformMesh.py#L192

The goal of axial mesh decusping is to honor important material boundaries in the uniform computational mesh; typically, these are the top and bottom of the fuel and absorber materials. The boundaries can be honored exactly as long as they are separated by more than the uniformMeshMinimumSize. When they are not separated by at least this distance, then the mesh from one of the material boundaries will be eliminated entirely from the axial mesh. It would be preferable to simply snap to the minimum mesh size in this case rather than removing the mesh point entirely.

I don't have any proposed implementation for this yet, I just want to document the thought.

[jakehader]

Good idea @mgjarrett

[john-science]

@mgjarrett Any movement on this?