Covariant form as variable-coefficient problem using auxiliary variables

[tristanmontoya]

This potentially replaces #31 by implementing the strategy described in #46, where the geometric information needed by covariant-form fluxes and source terms is passed in as auxiliary variables. The cache is not passed around anymore.

[codecov]

Codecov Report

Attention: Patch coverage is 98.50746% with 5 lines in your changes missing coverage. Please review.

Project coverage is 87.91%. Comparing base (49ec206) to head (7e4a152).

Files with missing lines	Patch %	Lines
src/equations/shallow_water_3d.jl	90.47%	2 Missing ⚠️
...vers/dgsem_p4est/dg_2d_manifold_in_3d_covariant.jl	98.42%	2 Missing ⚠️
...em_p4est/containers_2d_manifold_in_3d_covariant.jl	98.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   84.71%   87.91%   +3.20%     
==========================================
  Files          11       17       +6     
  Lines        1125     1448     +323     
==========================================
+ Hits          953     1273     +320     
- Misses        172      175       +3     

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