[h5md-user] fields of observable group

[Felix Höfling]

Hi,

our current definition of an observable group is not consistent with  
respect to the use of singular/plural:

observables
 \-- obs1
 |    \-- samples
 |    \-- step
 |    \-- time

I suggest to use the singular form throughout, i.e. (sample, step, time).  
This would be in accord with the good practice to use singular identifiers  
for containers.

Regards,
Felix