Add first, working gravity solution

When run as

    ./test_impestfpa_ad permeability=1.01325e15 ref_pressure=1.0e-5 \
    mu1=1000 mu2=1000 rho1=1 rho2=1

this example reproduces the expected solution (a one-Pascal pressure
increase per cell).

Note that this is an incompressible case, so 'test_impestpfa_ad' must
be compiled with the pre-processor symbol

    HACK_INCOMPRESSIBLE_GRAVITY

defined to a true value (e.g., -DHACK_INCOMPRESSIBLE_GRAVITY=1)