[fix] fixed how the filestem is extracted in hadrons/read_hdf5.

[fjosw]

Closes #217

Can you check if this fixes your issue (and does not cause problems in other places) @jkuhl-uni ?

[jkuhl-uni]

Hi @fjosw,
thank you for the quick fix. This seems to at least circumvent my problem.
Of course, I cannot speak for others.
In the next few months, I could work towards building a test-suite for this module, that could bring some more clarity and security. Sadly, this is not a thing done quickly for me at the moment.