Save additional solver information for `DGMulti` to handle non-`Polynomial` approximation types

[tristanmontoya]

In #2259, we have added basic mesh I/O and the ability to save solutions for the DGMulti solver. When the corresponding Trixi2Vtk.jl PR trixi-framework/Trixi2Vtk.jl#103 is merged, it will be possible to visualize DGMulti solutions using the same toolchain as for DGSEM.

Currently, however, the solver information that is written to the mesh and solution files consists solely of the element shape and the polynomial degree. When reading a solution or mesh file, it is then assumed that the solution is stored as nodal values at the default interpolation nodes for a Polynomial approximation, and the solver's basis field, which is type StartUpDG.RefElemData, is "reconstructed" under such assumptions. Further information about the basis would be needed to be stored in the mesh/solution files in order to properly load meshes and solutions for non-Polynomial approximation types.

Note: In the case of triangular prism (i.e. Wedge) elements with a TensorProductWedge approximation type, a tuple polynomial degree is saved, and it is currently assumed that the solution is stored at a nodal set constructed from the Cartesian product of the default Polynomial interpolation nodes on the Tri and Line elements.

[tristanmontoya]

As discussed in today's meeting, we can try using JLD2 to save the basis to file at the start of a simulation.