Add solution_container and auxiliary_container for covariant DGMulti solver to match upstream refactor

[tristanmontoya]

I have kept the contents of solution_container in the top-level cache as well for compatibility with the released version of Trixi.jl in the meantime. This should not actually duplicate any data. This change should hopefully allow both TrixiAtmo.jl CI and Trixi.jl's downstream tests to pass, addressing #159 for real this time.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.09%. Comparing base (338e6bc) to head (d6ddfc8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #163   +/-   ##
=======================================
  Coverage   97.08%   97.09%           
=======================================
  Files          37       37           
  Lines        5010     5019    +9     
=======================================
+ Hits         4864     4873    +9     
  Misses        146      146           

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[benegee]

Thanks for the effort!

I checked this locally with the current main branch of Trixi and it worked!