startEarning()
Callable only by minter on the Approved TTG earners List.
Allows a user to start earning yield on their M balance. balanceOfuser will be rewritten to store principalOfEarningSupplyMToken,earner
Calculates principalOfEarningSupplyMToken,earner=balanceOfMToken,earner/indexnow
Saves totalPrincipalOfEarningSupply+=principalOfEarningSupplyMToken,earner
Saves totalNonEarningSupply−=balanceOfMToken,earner
Saves balanceOfMToken,earner=principalOfEarningSupplyMToken,earner
Sets isEarningMToken,earner=true
Calls updateIndex.