make forms work more nicely with MeshSequence

[mscroggs]

If a MeshSequence is used for meshes with multiple cell types (eg hexes and prisms), currently forms need to be written like this:

u_hex, u_prism = ufl.TrialFunctions(W)
v_hex, v_prism = ufl.TestFunctions(W)
prism_domain, hex_domain = py_mesh.ufl_domain().meshes
dx_hex = ufl.Measure("dx", domain=hex_domain)
dx_prism = ufl.Measure("dx", domain=prism_domain)
a = ufl.inner(ufl.grad(u_hex), ufl.grad(v_hex)) * dx_hex
a += ufl.inner(ufl.grad(u_prism), ufl.grad(v_prism)) * dx_prism

This PR updates UFL so that this form can be written like this:

u = ufl.TrialFunction(W)
v = ufl.TestFunction(W)
a = ufl.inner(ufl.grad(u), ufl.grad(v)) * dx

The form returned in the new example is the same as in the more complex example above.

This only impacts a code path which previously threw an exception, so this should not affect and code that currently works.

Still to do:

• Add a test