[h5md-user] Restrictions on observables group

[Felix Höfling]

Hi all,

The section "general organization" says that the ``observables`` group  
"contains all time-dependent variables in the system, except the ones  
found in the ``particles`` group."

I find this statement too restrictive in 2 directions:  It forbids any  
other root groups in addition to "particles". Second, it forbids to store  
time-independent observables (which for some applications might be a  
useful option).

I suggest to rephrase without excluding additional data sets or root  
groups:

The ``observables`` group contains data of physical observables, i.e.  
quantities that are derived from the system state at given points in time.

What do you think?

Best regards,

Felix