Support spherical shell blocks, create Sphere domain

[nilsvu]

Proposed changes

Add an enum to Block that indicates whether the block represents a Cube or a SphericalShell. Cubes can only have one block neighbor per dimension, whereas spherical shells can have multiple block neighbors (another shell, or 4 wedges in 2D, or 6 wedges in 3D).
Also adds the code to connect element neighbors across spherical and cubed shells, to create the Ylm meshes, and to create a Sphere domain with spherical shells.
Only the spherical-to-cartesian coordinate map is missing to create spherical shells in ExportCoordinates3D. The coordinate maps are in #5588, but need to be adjusted a bit because we use the spherical coordinates directly as logical coordinates, so we don't need the maps from [-1, 1] to angles.

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• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
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Further comments

[nilsvu]

@kidder this is to share this code with you. I have cleaned up the first 3 commits. The others need some more cleanup. I can do that and add some more tests. The last commit e4c2593 will be superseded by your #6389 .