Remove salinity from tracers

[simone-silvestri]

Ice salinity can be prognostic or diagnostic or prescribed, thus it is not a tracer because tracers are assumed to obey an evolution equation for which we have tendencies (i.e. always prognostic), therefore it is more sustainable to extract out salinity so that we can make general assumptions on the fields held in the tracers field (i.e. that we have to compute a tendency and we need to evolve them).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.72%. Comparing base (c6aa25c) to head (b877143).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
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- Coverage   70.76%   70.72%   -0.04%     
==========================================
  Files          27       27              
  Lines         797      796       -1     
==========================================
- Hits          564      563       -1     
  Misses        233      233              

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[glwagner]

I was thinking that we can have an abstraction for diagnostic tracers like we have for background fields. So any tracer can be prognostic or diagnostic.

The nice thing about this is that the place where you'd find salinty S isn't moving around in the model struct depending on configuration. You can count on model.tracers.S whether S is diagnostic or prognostic.

What design do you propose instead?