Add wet/dry functionality for the 2D Multilayer-SWE

[patrickersing]

Analogous to #38 this PR extends the ShallowWaterMultiLayerEquations2D with functionality to deal with wet/dry transitions.

[codecov]

Codecov Report

Attention: Patch coverage is 99.06103% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 97.41%. Comparing base (a684879) to head (132e008).

Files	Patch %	Lines
...c/callbacks_stage/positivity_shallow_water_dg2d.jl	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   99.23%   97.41%   -1.83%     
==========================================
  Files          54       58       +4     
  Lines        2228     2436     +208     
==========================================
+ Hits         2211     2373     +162     
- Misses         17       63      +46     

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[andrewwinters5000]

Nice work @patrickersing ! Overall everything looks good to go. Just a few small corrections in the comments

[andrewwinters5000]

LGTM!

[andrewwinters5000]

We again get a few uncovered lines that should be reported as covered by testing (similar to the discussion here). We can still merge this into main but we can keep an eye on these strange lines...

[patrickersing]

I don't think we have to worry about these uncovered lines. Looking at the coverage reports from Trixi.jl, there are numerous untracked end lines when @threaded-loops are used. Since only the end is missing coverage, we can also be sure that everything is tested. So I think we can go ahead and merge this.