Optional unit with some trade codes

this fixes 2 bugs related to the fact that unit should be allowed to be blank with some trade codes (e.g. BOD) but not with others (e.g. CAV)

To test you need to reimport acceptable trade code pairings:
in psql console:
delete from term_trade_codes_pairs;

in shell console:
rake import:trade_codes_t_p_pairs
rake import:trade_codes_t_u_pairs