Adding a netgen flag to specify the index value associated with each label

[UZerbinati]

Adding a netgen flag to specify the index value associated with each label.
Here is a minimal working code.

from firedrake import *
from firedrake.output import VTKFile
from netgen.occ import *

rect=WorkPlane().Rectangle(2,2).Face()
circ = WorkPlane().MoveTo(1,1).Rectangle(0.5,0.5).Face()
rect.edges.name = "outer"
circ.edges.name = "inner"
shape = rect-circ
geo = OCCGeometry(shape, dim=2)
ngmesh = geo.GenerateMesh(maxh=1.)
mesh = Mesh(ngmesh, netgen_flags={"labels": {"outer": 1, "inner": 2}})
V = FunctionSpace(mesh, "CG", 1)
f = Function(V)
DirichletBC(V, -1, [1]).apply(f)
DirichletBC(V, 1, [2]).apply(f)

vtk = VTKFile("output/test_labels.pvd").write(f)

[pefarrell]

I don't know. This seems misguided. I would prefer the interface where the user code would look like

DirichletBC(V, 0, "outer")

and for the user to forget about the integer labels altogether. @rckirby , what do you think?

[rckirby]

This PR is motivated by a use case where I'm hooking up to an external operator that can only do its thing on a boundary if all the boundary segments of interest share the same integer label. So your cleaner interface is cleaner but...

Does anything stop us from using this PR to relabel all "inner" as "inner" rather than 1, that is, the first "inner" corresponds to sets of boundary segments in the netgen geometry and the second refers to labels that we attach to those edges in the mesh?

[UZerbinati]

This PR is motivated by a use case where I'm hooking up to an external operator that can only do its thing on a boundary if all the boundary segments of interest share the same integer label. So your cleaner interface is cleaner but...

Exactly the issue is that Netgen will number differently entities with the same labels, what we do here is to force all entities with the same label to have a given id in the DMPlex when conversion happens.

Does anything stop us from using this PR to relabel all "inner" as "inner" rather than 1, that is, the first "inner" corresponds to sets of boundary segments in the netgen geometry and the second refers to labels that we attach to those edges in the mesh?

Can we attach a label that is a string to the entities of a DMPlex, I only have seen Firedrake using integers.