Start by implementing pressure diagnostic.  Addressing issue #280.

Conflicts:
	src/base/stressbalance/PISMStressBalance.cc
	src/base/stressbalance/PISMStressBalance_diagnostics.cc
	src/base/stressbalance/PISMStressBalance_diagnostics.hh
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.

Conflicts:
	src/base/stressbalance/PISMStressBalance.cc
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.

Conflicts:
	src/base/stressbalance/PISMStressBalance_diagnostics.cc
	src/base/stressbalance/PISMStressBalance_diagnostics.hh
Cleaning up after cherry-pick (last three commits).  Done with #280 in stable0.6.