Allow non-sequential boundary ID values

[aprilnovak]

nekRS currently requires that the boundary IDs be ordered sequentially beginning with 1. However, many mesh generation tools do not impose such a requirement, necessitating extra care from the user to correctly set up valid boundary IDs for nekRS. It would add flexibility to nekRS to allow arbitrary integer boundary IDs.

Desired solution
Update the boundaryTypeMap field to take a tuple of boundary IDs + boundary character type, such as

boundaryTypeMap = 200, v; 300, w; 400, o

where 200, 300, and 400 are arbitrary integer boundary IDs in the mesh, and v, w, and o are typical character boundary type identifiers in nekRS.