Support string subdomain_id in ufl.Measure, DirichletBC and Submesh

[pbrubeck]

Description

Disclaimer: This PR was implemented with Codex

Introduces native, human-readable region naming capabilities across Firedrake. By allowing users to seamlessly use string identifiers preserved from external mesh generators or explicitly assign custom semantic names to subdomains at runtime, it eliminates the cognitive overhead of tracking opaque numerical tags when defining boundary conditions, extracting submeshes, or specifying UFL variational forms.

from firedrake import *
import netgen.occ as ngocc

# 1. Generate geometry & name boundaries natively in Netgen
wp = ngocc.WorkPlane()
box = wp.Rectangle(1, 1).Face()
box.edges.Max(ngocc.Y).name = "inlet"
box.faces.name = "fluid_domain"
ngmesh = ngocc.OCCGeometry(box, dim=2).GenerateMesh(maxh=0.1)

# 2. Native Firedrake Mesh captures the names automatically
mesh = Mesh(ngmesh)

# 3. Explicitly map a single string name to multiple disjoint entity IDs

# 4. Clean, human-readable API usage
V = FunctionSpace(mesh, "CG", 1)

# DirichletBC automatically maps "inlet" to the correct face ID
bc = DirichletBC(V, 0.0, "inlet")  

# SubMesh automatically maps "fluid_domain" using the correct keyword argument
submesh = Submesh(mesh, subdomain_id="fluid_domain")

# Form language integration: UFL Measures accept string region names directly
f = Function(V)
L = f * dx(subdomain_id="fluid_domain")