Please write proper unit in PI-timeseries export (applies to version 2.3 and higher)

Currently pi_mixin writes unit_unknown in the timeseries header during export.

please write a proper unit if known based on the SI specification as used in the Modlica model. Currently both flow and volume, typically expressed in m3/s and m3, have the same unit_unknown, which gives trouble on the Fews-side when I have to convert this to ML/day (multiply by 86.4) vs. ML (multiply by 0.01)

Please fix in version 2.3 and higher