Expand the used to MixedFSOptions to one time discretisation used on the whole equation set

[atb1995]

Currently, if you try to used MixedFSOptions on the whole equations set it fails.
Example code is:

suboptions = {'b': EmbeddedDGOptions()}
opts = MixedFSOptions(suboptions=suboptions)
transport_methods = [DGUpwind(eqns, "u"),
                     DGUpwind(eqns, "p"),
                     DGUpwind(eqns, "b")]
scheme = SSPRK3(domain, options=opts)
! Time stepper
stepper = Timestepper(eqns, scheme, io, transport_methods)

Error is: raise RuntimeError("%r defined on incompatible FunctionSpace!" % r)