Better handling of units

[grzanka]

This should not be necessary, since they are already written as text strings in the bdo file?

Originally posted by @nbassler in https://github.com/DataMedSci/pymchelper/pull/348

[nbassler]

Based on the #372 discussion we had, I think the idea is to

• use units embedded in .bdo file, if available
• otherwise fall back to pymchelper internal unit lists.

This keeps the compability to FLUKA.

So, with #372 this is actually closed?

[grzanka]

Yes, should be closed now