Allow ballast or use vehicle minimum weight as alternative to minimum weight for cat-back

[barm]

Rijenz was bumped to Street Class due to an cat-back that was below the minium weight. He would like to keep his exhaust but still prefers to participate in Stock Class.

[barm]

or:

• remove stock class catback weight limit
• replace weight limit with attribute-based rules: e.g. must have muffler and dual exits