One can now attach subdomains to a form:
a.cell_domains = cell_domains
a.exterior_facet_domains = exterior_facet_domains
a.interior_facet_domains = interior_facet_domains
As a consequence, one does no longer need to send these arguments to
the assembler. Another consequence is we can simplify the
VariationalProblem class and remove the extra functions that contain
these arguments (will fix tomorrow). It also means one can use
VariationalProblem for cases where different domains are needed for a
and L.
It is still possible to send the arguments to the assembler so it's
backwards compatible. It is also (now) possible to specify the domains
using MeshData with names "cell_domains", "exterior_facet_domains" and
"interior_facet_domains".
Arguments to the assembler will override subdomains attached to a
form which in turn overrides subdomains specified as part of the mesh.
