Draft: Radial shell output

This MR implements output of FE coefficients (P1 and P2, scalar and vector-valued) to point clouds that are organized in radial shells for the thick spherical shell mesh (with blending).

The scope is as follows:

• a new class RadialShellData to gather and store the FE data in radial layers
• VTK output for point clouds (general point clouds - including those generated above) via VTKPointCloudOutput
• test P2 since edge DoFs and shells must somehow be organized meaningfully
• ADIOS2 output (requires ADIOS2 consulting from @mohr)

It hopefully will also resolve issues regarding the nomenclature of shell and layer as well as corresponding computations. It appears that their definition is unclear. We need to distinguish between

• (what I currently call shell) 2D manifolds in 3D space, i.e., the boundaries of a sphere (centered at the origin) with a specific radius, and
• (what I currently call layer) 3D volumes that have two such manifolds as boundaries.

Unfortunately, these definitions are not used consistently, even in older code and documentation.

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VTK seems to work already for P1:

Example 1: all shells selected:

Example 2: only two selected shells: