[h5md-user] H5MD author and creator attributes

[Peter Colberg]

Hi all,

The following issue is a simple one.

While pondering over the issue of the unit attribute, I thought that
it would be a good idea to prepare the h5md group for the addition
of future meta-data.

If one wishes to add further attributes to the author (e.g., their
organization) or to the creator (e.g., its variant or git branch),
the h5md group becomes crowded with underscore names of the form
author_<attribute> or creator_<attribute>.

I propose to modify the structure of the h5md group as follows:

    h5md
     \-- author
     |    +-- name
     |    +-- (email)
     +-- creator
     |    +-- name
     |    +-- version
     +-- version

The rationale is to bundle meta-data that describes the same object
(or subject, for the case of author) into a common group, rather
than prepending a common prefix to their attribute names.

Peter