Implement the Cubed Sphere as P4estMesh

[efaulhaber]

10th point of #584.

[codecov]

Codecov Report

Merging #689 (bcf9274) into main (3ae92e6) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
+ Coverage   93.63%   93.70%   +0.07%     
==========================================
  Files         171      171              
  Lines       16535    16712     +177     
==========================================
+ Hits        15482    15659     +177     
  Misses       1053     1053              

Flag	Coverage Δ
unittests	93.70% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
src/meshes/p4est_mesh.jl	98.39% <100.00%> (+0.75%)	⬆️

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[sloede]

LGTM in general! just a few suggestions/questions/remarks from my side, but no blockers