ZEVA2 - ZEV unit models and structures

Implement ZEV unit data models (transactions, transfers, rollover balances) and structures (classes?) that encapsulate the section 12 offsetting rules, as well as the compliance reduction obligation/section 16.1 reduction rules. The goal is to create a sort of "API" that allows developers to work with ZEV units (e.g. totalling them, offsetting them, deriving a balance from them, etc.).

Also write some documentation for this API and add it to the developers' guide.