[h5md-user] unit attributes

[Felix Höfling]

Hi,

I realised that the optional attribute "unit" is only mentioned for the  
"value" datasets in the trajectory group. Actually, the "time" dataset may  
also carry a unit. The same applies for the groups in /observables.

I would like to generalise and to modify the value/step/time structure of  
data groups as

data_group
 \-- value
     +-- unit
 \-- step
 \-- time
     +-- unit

Felix