p4est solver on a spherical shell

[amrueda]

This PR implements a version of the 2D p4est solver that runs on a spherical shell that is constructed from a cubed sphere. Since there are three Cartesian space dimensions on the surface of the sphere, we use the 2D p4est solver with a 3D equation.

The connectivities are inherited from the p8est 3D cubed-sphere implementation, but they are adapted to p4est. The metric terms are computed with a cross-product form, as it is standard for the DG implementations of the shallow-water equations on the surface of a sphere. See, e.g.,

• Giraldo (2001). A spectral element shallow water model on spherical geodesic grids. http://faculty.nps.edu/fxgirald/Homepage/Publications_files/Giraldo_IJNMF_2001.pdf.

earth.mp4

The following points need to be improved (more information in the threads below):

• The current implementation of get_contravariant_vector causes too many allocations.
• Improve call to new source term that depends on du.

[github-actions]

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