New cubed sphere 2 grid

[mo-jonasganderton]

This PR has been moved to ecmwf/atlas.

Description

Initial implementation of a new cubed sphere grid.

@odlomax and @wdeconinck can both have a look over to recommend any changes - I expect this will have changed once everything else has also been implemented.

Using this implementation generates the following lonlat points - n=12, red=tile0, purple=tile5, darkest point of each colour is tij=0, lightest point is tij=143. This matches the sequence of the points in the LFRic cubed sphere.

Impact

Once everything else is also implemented, this makes the cubed sphere more maintainable.

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have run the unit tests before creating the PR

[codecov]

Codecov Report

Attention: Patch coverage is 64.23358% with 49 lines in your changes missing coverage. Please review.

Project coverage is 79.95%. Comparing base (e734e7a) to head (4e288ad).
Report is 161 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/atlas/grid/detail/grid/CubedSphere2.cc	56.66%	26 Missing ⚠️
src/atlas/grid/detail/grid/CubedSphere2.h	58.33%	20 Missing ⚠️
src/tests/grid/test_cubedsphere_2.cc	88.46%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #96      +/-   ##
===========================================
- Coverage    80.02%   79.95%   -0.08%     
===========================================
  Files          857      834      -23     
  Lines        63498    64805    +1307     
===========================================
+ Hits         50817    51814     +997     
- Misses       12681    12991     +310     

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