Tweaking the SWIG interface file.
Start passing IceGrid by const reference (or pointer).
Add Component::get_grid().
Add IceModel::get_grid().
Diag and TSDiag constructors don't need to take IceGrid.

(Every model has a grid, so we can just use ...->get_grid().)
Component constructor takes "const IceGrid&".
More const-ness.