Enhance axom::bump::MergeMeshes with mixed-field support

[BradWhitlock]

Blueprint supports the notion of mixed fields that allow a zone to contain multiple values for the field, 1 per material fragment in the zone.

The axom::bump::MergeMeshes class in Axom (and likely others) lacks support for merging multiple meshes that contain mixed-fields.

Mixed Fields:
    fields/field/association: “vertex” | “element”
    fields/field/basis: (mfem-style finite element collection name) (replaces “association”)
    fields/field/volume_dependent: “true” | “false”
    fields/field/topology: “topo”
    fields/field/values: (mcarray)
    fields/field/matset: “matset”
    fields/field/matset_values: (mcarray)

The main thing that is not yet supported is merging the matset_values array which is where the per-material-fragment values are stored.

• Add support for element associated material-dependent fields (what the heck would a vertex-associated one be?)
• Create some new tests that demonstrate merging 2+ domains with such fields
• Update RELEASE-NOTES.md
• Identify other bump classes that may contain the same issue and note them in a new issue

[BradWhitlock]

I'm pursuing an idea I had about reusing the MatsetView types to traverse the mixed field data since their representation changes depending on the structure of the matset.