Fix Courant number calculation

[tommbendall]

Fixes the Courant number calculation which is breaking tests following this Firedrake PR: firedrakeproject/firedrake#3947

[tommbendall]

Unfortunately there seem to be other failures here...

1. boundary_layer_mixing tests are failing with ufl.algorithms.check_arities.ArityMismatch

2. test_zero_limiter.py and some thermal shallow water tests, failing with a petsc error in the linear solve

3. test_moist_compressible, test_dry_compressible, test_simultaneous_SIQN (order 1), test_boussinesq.py (compressible) all failing with changed KGOs

4. test_tracer_conservative_transport no longer passing consistent transport test

[jshipton]

Rerunning because the boundary layer test works fine on my laptop after a Firedrake update...

[jshipton]

Rerunning because the boundary layer test works fine on my laptop after a Firedrake update...

Ah well.

[tommbendall]

I think we're confident that we need these fixes, and I'd like to get a clearer picture of our remaining errors so I'm going to just merge this as is