SubMesh of multiple subdomains

[Nico Schlömer]

The syntax for extracting a submesh from a mesh is

submesh = SubMesh(mesh, subdomains, 24)

Sometimes, it is necessary to combine two or more subdomains into one submesh, e.g.,

submesh = SubMesh(mesh, subdomains, [24, 25])

See http://fenicsproject.org/qa/513/submesh-of-two-subdomains.

[Jan Blechta]

Analogical feature would be useful also for DirichletBC.

[Martin Sandve Alnæs]

Same for Restriction, or whatever else Restriction is replaced with if it's replaced.

[Martin Sandve Alnæs]

In other words, it would be nice if all places taking a subdomain id can take a tuple just as f*dx((24,25)).

[Prof Garth Wells]

Marking this a won't fix because it could be incorporated into MeshView class. The name SubMesh is misleading.

See https://bitbucket.org/fenics-project/dolfin/issue/329