save units in attrs of H5

[PythonFZ]

• fix compliancy with H5MD by using attrs
• add MDAnalysis test to read file (see H5MD Can not read observables/<group>/<observable>/... MDAnalysis/mdanalysis#4598 for why the test is so minimal)
• deprecate initialize_database_groups
• now downstream issues with IPSuite test if znh5md update works IPSuite#300

[M-R-Schaefer]

looks good. But this will break existing IPS H5 files, right? Is there some sort of backward compatibility functionality?
If not it?s fine too, I just need to breace myself 😂

[PythonFZ]

looks good. But this will break existing IPS H5 files, right? Is there some sort of backward compatibility functionality? If not it?s fine too, I just need to breace myself 😂

Reading should be fully backwards compatible