List the reviewed conversions from a BOM line's unit to a factor's unit

For a line whose unit is in another dimension than the factor's (m² of fabric against a per-kg factor), the approved conversion rows of the active set, in the precedence item, then the factor's reference product, then its material class. Several options are variants the user chooses between. An empty list means no reviewed value exists and the user enters one. Values are never estimated. Requires org.read.