Start by implementing pressure diagnostic.  Addressing issue #280.
This works.  It is inelegant because it does the computation in PSB_pressure not in the ShallowStressBalance where the hydrostatic approximation already is assumed.  My way of doing this may not generalize to tauxz,tauyz because I may not be able to get access to driving stress.  See issue #280.
All diagnostics pressure,tauxz,tauyz in #280 added.  Note (1) they are shallow formulas at StressBalance level, and (2) there is code duplication between PSB_tauxz and PSB_tauyz.