Ensemble methods dispatch to MPI methods for non-Firedrake types

[JHopeCollins]

This means the call site for Function/Cofunction looks the same as the call site for non-Firedrake types.
Currently to send a non-Firedrake type:

x = ensemble.ensemble_rank
x = ensemble.ensemble_comm.bcast(x, root=0)

This PR changes it to:

x = ensemble.ensemble_rank
x = ensemble.bcast(x, root=0)

[JHopeCollins]

The CI failure is an unrelated linkcheck

[connorjward]

Just some tiny suggestions. Otherwise happy.