Trouble with new ridging

This is what I get in the NEP7 domain with ridging turned on:
```
FATAL from PE    88: SIS_tracer: register_SIS_tracer must be called for salin_ice before get_registered_tracer_pointer is called for it.
```
In ice_model.F90, salin_ice is only registered if ice_rel_salin is changed from the default of 0. The ice ridging doesn't have that check before asking for the tracer pointer.