1. (Experimental): Supply Neumann top surface boundary condition to PISM.

2. Accumulate variables relevant for two-way coupling in each PISM timestep (enthalpy or mass).

3. Moved access to protected vars from IceModel_PISM to PISMIceModel (which subclasses PISMs IceModel), thereby eliminating need for accessor functions in PISM.

4. Synchronize time between GLINT2 and PISM.