Clean up energy command-line options. See #144.

Use "-energy none" to disable energy conservation, "-energy cold" to use
cold ice methods, "-energy enthalpy" to use the default, enthalpy-based
model.

Also:

- remove -ssa_floating_only and the "use_ssa_when_grounded" flag.
- PISMMohrCoulombYieldStress no longer works when hydrology_tillwat_max is
  zero (use "-yield_stress constant -tauc 1e6" or similar).
- Remove IcePSTModel.

Next up: cleaning up stress balance command-line options.