Hexagonal Lattice Roundtrip

[pshriwise]

Description

This fixes a bug described in #3002 where hexagonal lattices don't round trip in and out of XML correctly. A break condition in HexagonalLattice.from_xml_element was occurring before the index into the flat array of universe IDs is incremented, causing the next universe ID used for the second axial to be the last universe from the first layer. The universes in the resulting HexagonalLattice object are scrambled. This is a silent bug unless one of the universe IDs is omitted from the new lattice entirely in which case it's possible (but not guaranteed) that the abandoned universe will be read in later as part of the cell parsing and treated as the root universe of the geometry.

I've added a test to check that the universes of a multi-layer hex lattice are roundtripped correctly both: a) when the universes in each layer are the same and b) when the universes for each lattice cell are unique.

Fixes #3002

Checklist

• I have performed a self-review of my own code
• ~ I have run clang-format (version 15) on any C++ source files (if applicable)~
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks for the fix and the nice test!