[h5md-user] fix remaining imprecisions

[Felix Höfling]

Hi,

I worked through the complete draft and I think we are close to the next  
release candidate and a final release (before Christmas?!). There are a  
few imprecisions which we should clarify in the spec.

I have added two sentences (explanations in the commit messages) and I  
have two open issues:

* 6152815 - particles/species: make meaning of 'species' more precise
* 3f08548 - particles: mention that subgroups are independent

* For "id" we should state whether the id is unique within the subgroup or  
across the whole particles tree.

* For "position", I can't recall the criterion whether the data are  
interpreted as "wrapped" or "unwrapped". This should be stated clearly.  
(BTW, would "folded coordinates" be better instead of "wrapped  
coordinates"?)

Please check the commits carefully, share your thoughts, and vote.

Regards,

Felix