Install firedrake against existing libraries

[jwscook]

• PETSc <- most important --honour-petsc only works for Firedrake's PETSc
• mpi <- done
• python <- done --honour-python

so that we have a toolchain that includes firedrake, rather than allowing firedrake to be the top level thing.

We would like spack / module to control the dependencies - need to bring this up with the Firedrake devs.

[jwscook]

Bring this up at the Firedrake meeting...