Remove IceModelVec2S::view_matlab().

MATLAB can read NetCDF files, so there is no reason to use ASCII.
SSA::m_velocity_old -> SSAFD::m_velocity_old.

m_velocity_old is used to re-start Picard iterations. SSAFEM does not
use it.
Add an assertion (PISMRoutingHydrology::V has to have ghosts).
Use a "global" Vec in SIAFD_topgsmooth::compute().

(This way it can be written to a file without making copy.)