See -dm_plex_gmsh_use_regions.
https://gitlab.com/petsc/petsc/-/issues/689#note_837942276
There are a couple issues. One is that a number of functions take a label and a
list of values. For example, we have meshes of a bunch of sand grains (e.g.,
segmented from CT images), where each grain is
PETSc developers,
Is there a reason why the PETSc Gmsh reader does not just set the PETSc label
name to the name of the physical group and set the PETSc label value to 1?
>From a user perspective, it would be nice to only have to specify one
>identifier for a physical group (preferably a
