Improve IceModelVec2::{get,set}_component(...)
Syntactic sugar (add PISMDM::operator DM()).

PISM's PISMDM is automatically converted into PETSc's DM if
needed. (Saves a little bit of typing.)
Silence SWIG warnings.
Generalize IceModelVec2::{get,set}_component().

Now they can be used to get and set several records at once. Takes
advantage of PETSc's contiguous storage of DOF corresponding to the same
grid point.
Remove last DMLocalToGlobalBegin/End calls. Port to 0.6.

This achieves compatibility with PETSc 3.5. (DMLocalToGlobalBegin/End is
broken in PETSc 3.5.)