Provide function that loads an HDF5 file and returns the solution interpolated to equidistant points

Quoting @ranocha:
> One feature I would like to have in 1D concerns visualization: Instead of using `trixi2txt` from `utils/trixi2txt.jl` to write a `.txt` file and read that again in Julia to plot something, could we just have a function returning the respective `x` and `u` arrays directly? Then, users can just create their own plots in Julia directly without having to deal with another set of files.