human readable files: all fields put in rather than just measured fields

[rerpha]

#24 introduced a new format in which the human-readable files are output as instrument_axis_date_time but currently the writer just puts all fields specified as the axis part rather than just the measured fields.

You can get these from the start message from "detectors" and "motors" (sometimes, we should account for there being no motors as count() for example won't put them in the start message)