Fix derivation of `price_ref` from `PRICE_EMISSION` about message_ix HOT 1 OPEN

To add some more detail from our Slack conversation:

• This occurs in the specific case where the set of MESSAGE commodity codes with level="useful" include ‘extra’ codes (in this case, "non-comm") that do not have corresponding MACRO sector codes.
• This results in values of PRICE_EMISSION corresponding to those ‘extra’ commodities to be retained, when they should instead be dropped.
• @macflo8 tested a fix that adds an explicit filter on commodity; this appears to work.

To resolve the issue, we:

• Insert this fix.
• Check that existing tests pass.
• Add additional tests for this case.
• Adjust or update documentation as necessary.

from message_ix.