Gmsh reader with Physical Entities

[smelchio]

Before the section $Nodes, a file from gmsh typically have a section such as

$PhysicalNames
2
3 1 "outer"
3 2 "inner"
$EndPhysicalNames

This is currently ignored by the gmsh reader. In
https://github.com/SNLComputation/omega_h/blob/master/src/Omega_h_gmsh.cpp#L268-L270
the physical tag is read but not stored. How would you suggest to do it? Could this information be stored in a csv like structure where a pair (a2ab, ab2b) would point from each physical entity a to all the tets b that are in it?

[ibaned]

I think we could use Omega_h::Mesh::class_sets to express this, and change classification to be based on physical Gmsh entities