Create trixi2txt postprocessing script to convert HDF5 files to text

In GitLab by @sloede on Mar 27, 2020, 07:05

This can be helpful if Trixi output data is needed for postprocessing by other programs. It should basically write CSV files in the following format:
```
# filename = "solution_0000123.h5"
# time = 1.2345
# polydeg = 4
#
x y rho u v
0.0 0.1 1.0 0.0 0.0
0.0 0.1 1.0 0.0 0.0
...
```

`x` and `y` refer to the node coordinates. In case of solution files with element variables, these should by default not be written (as they would have a different length). Provide a `--separate-celldata`/`-s` flag to write this data to a separate file instead, using the element coordinates (not node coordinates!) for `x` and `y`.

Mesh files should be handled in a similar way.