Allow preprocessing solution data based on coordinates in `SaveSolutionCallback`

It is currently possible to pass something other than `cons2prim` to `SaveSolutionCallback`, so custom quantities can be visualized by Trixi2Vtk.
However, in a custom `cons2prim` function, only the solution data is available and not the coordinates. It would be very convenient to allow preprocessing of the solution data with access to the coordinates directly from an elixir. This is, for example, useful when a simulation is done on the cubed sphere, and one wants to show the velocities in spherical coordinates.