Allow any point-wise transformation of the output variables in solution files

[sloede]

I think in restart files we should continue to only store the conservative variables, as they are the only guarantee that we will not have any floating point errors due to a restart.

As for the solution files, currently we support "primitive" and "conservative". I think it would be nice to have the option to supply arbitrary transformation functions (that can also be user-supplied), to store, e.g., characteristic variables, entropy variables etc. Since we already have cons2prim, cons2entropy and, with #172, cons2cons, not much additional code would be required, except a mechanism to also supply different variable names.