Remove pism::Component::init().

Different components may need different arguments.
IceModelVec methods that modify the stored field call inc_state_counter().

This will let us detect if a field was initialized, for example.
Cleanup.
Address #291.

We use IceModelVec::get_state_counter() to check if we need to use
heuristics to fill 3D fields during bootstrapping.

(IceModelVec::regrid() calls IceModelVec::inc_state_counter(), so we can
save this counter, try to regrid, and then compare.)