load_file() ignores "Unit: …" comment

[khaeru]

iiasa/message_data#529 appears to be due to a file that contains a "# Unit: …" comment in its header (here, related to iiasa/message_data#522), but which is ignored by a direct call to genno.operator.load_file(). The Quantity that results is dimensionless instead of having the stated units.

@ravitby and I suspect this could be due to some non-printing characters or other issues in the file which prevent the unit line from being recognized or parsed correctly.

As a mitigation, we gave the explicit keyword argument load_file(…, units="Gp km /a"), which seemed to resolve the error due to the missing units.