Separate functions for retire and burn in emissions audits and offset tokens

[sichen1234]

In the past we've used the burn function in NetEmissionsTokens.sol to retire emissions audits and offsets. This means that the offsets cannot be transferred again. Emissions audits are automatically retired so they can never be transferred to someone else.

I think we should create separate retire and burn functions. Retire will continue to mean that the offsets are used, ie not transferred again. Audits will continue to be retired. The retired balance of the offset is the amount of offset used to offset one's emissions, and the retired balance of emissions audits is the amount of audited emissions.

Meanwhile burn will mean that the audit or offset is invalid. The retired balance will be set to zero when a token is burnt. This means that the owner will have to purchase new offsets to replace them, or get a new emissions audit to replace the one that was invalidated.

Burning the tokens will be reserved for the contract owner for now. It should involve some group consensus vote (ie through a DAO). If tokens are burnt, it should cause loss of reputation tokens for its proponents.