Implement weak div perp test unit sphere

[amartinhuertas]

This PR implements, among others, the DIV and Laplacian on the unit sphere manifold using AD ... as defined here

where (u=(u1, u2) are the coordinates in parametric space, x the mapping, G = J^T J, J=dx/du)

Besides:

• [x] Fix bug in get_normal_vector. (it is flipped for some panels)
• [x] Systematize convergence study of weak div perp operator (parametrized by mesh, FE order, etc.)

[codecov-commenter]

Codecov Report

Merging #4 (b0c45da) into master (7c0d9c2) will increase coverage by 8.14%. The diff coverage is 69.64%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   32.69%   40.83%   +8.14%     
==========================================
  Files           6        8       +2     
  Lines         208      262      +54     
==========================================
+ Hits           68      107      +39     
- Misses        140      155      +15     

Impacted Files	Coverage Δ
src/CubedSphereTriangulations.jl	27.77% <ø> (+2.77%)	:arrow_up:
src/CoordinateTransformations.jl	37.50% <37.50%> (ø)
src/Operators.jl	68.18% <68.18%> (ø)
src/CubedSphereDiscreteModels.jl	97.59% <100.00%> (+0.66%)	:arrow_up:

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