PETSC_NULL is deprecated. Replace with NULL.
Cosmetic changes.
Cleanup.
Add the energy_advection_ice_thickness_threshold parameter.

This should make it easy to test if ignoring temperature (enthalpy)
advection and strain heating makes a difference in verification tests F
and G. See #292.
Add m_ prefix to protected data members of IceGrid.

Eventually IceGrid should implement "getters" and *all* its data members
should use this prefix.
Update the LocalInterpCtx documentation comment.
Add "using std::vector" and "using std::string" to PIO.cc

It's OK to import *particular* objects from the std namespace into the
current namespace in .cc files.
Add PIO::use_mapped_io().
Cleanup in PIO.cc