BOUT++ has MUMPS

Configure script now tries to find MUMPS if given --with-mumps
option. This currently looks to see if PETSc has downloaded and installed
MUMPS and its dependencies (e.g. ParMetis, Scotch), which is
the easiest way to get MUMPS.

Quite fragile still, and doesn't properly handle all the linker flags. Works
on York machines, but probably not many others.