Improved comment
(1) Removed get_geothermal_flux() and other similar methods, (2) Added massContPostHook()
1. Users who need access to these variables should subclass IceModel instead.
2. massContPostHook() is called in IceModel::step().
Allows for subclasses to access and sum conservation-related information
on a per-timestep basis.
Added sumColumns() as a class method for IceModelVec3
See https://github.com/pism/pism/issues/229