Bugfix: convert array to matrix type for matrix multiply.
Fixes in computeFluxes().

 - Give AD variables the correct block pattern (although as we
   change fluid interface they will not need to be ad types).
 - Make sure 'flux' is only on inner faces.
 - Copy full fluxes to state.faceflux().
Print face fluxes after solve.

Also define the HACK_INCOMPRESSIBLE_GRAVITY symbol, since test case
is meaningless without it.