csv2hdf5: improve usability of conversion script

[20DM]

This is to make the script more command-line-friendly.

[tkoskela]

I didn't have all the dependencies in my python env. Should this come with a requirements.txt/pyproject.toml or something like that?

[20DM]

This is an optional utility script, not a fully fledged Python package, so no to requirements.txt/pyproject.toml, but I've clarified the error message when hdf5py cannot be found. If the user wants to write out an HDF5 file with a Python script, having hdf5py installed would help of course.

[tkoskela]

LGTM