Purge use of firedrake prefix in tests

We do from firedrake import * everywhere, which doesn't give us
firedrake.Foo.
Plex: fix mesh creation from cell lists in parallel

We should only pass in the topology data on rank zero, and then
distribute, rather than replicating the mesh on all ranks.
Raise error for periodic interval in parallel

Creation of mesh and replacement of coordinates only works in serial for
now.
Add some simple-minded parallel mesh tests

Compute the integral of one over various different meshes in parallel.
Remove ability to spawn gmsh process

Since we rely on gmsh for much less mesh generation now, and spawning a
gmsh process was so flaky, only support building meshes from .geo files
if we have gmshpy installed.

This additionally removes references to gmsh2triangle and fldecomp.
Skip unit circle tests if gmshpy not available