Many changes to support coupling contracts and transfer of constants between GCM and Ice Model.

1. Transfer constants from the GCM to the Ice Model, with unit conversions as appropriate.  No more constants in the GLINT2 config file.  Callback for use with ModelEs (new) set_all_constants() is provided.

2. Coupling contracts are set up between ModelE and PISM.  Enthalpies are converted as appropriate.

3. Modified ModelE API initialization procedure, to insert GCM constant discovery step into it.

4. Changed names of unique and shared pointer casts (giss/memory.hpp)

5. IceModel_PISM::process_options() was unused, and has been removed.