H5MD writer: write time-averaged data as (value,error,count)

Instead of passing the accumulator directly, we are more generic and
receive three data slots for the triple.

http://nongnu.org/h5md/discussion.html#time-averaged-data
observables.utility.accumulator: write data as one H5MD element

Rather than splitting the data in three H5MD elements (value,
error_of_mean, count), use a single element extended by error and count.
Thereby, the writer parameter "fields" is not needed anymore.

Fix output intervals: write first results _after_ 'every' steps, not at
the initial step already.

This structure has been suggested as an extension of H5MD 1.0.

http://nongnu.org/h5md/discussion.html#time-averaged-data