Support angular units in detector layout #847
Description
The x_cen and y_cen in the detector layout table are sometimes easier found in agular units, but putting e.g. arcmin in x_cen_unit or y_cen_unit results in UnitConversionError: 'arcmin' (angle) and 'mm' (length) are not convertible. It should be possible to simply convert these at the correct time via the plate scale.