update(TwabLib): uniform uint types for calculations

[kamescg]

No description provided.

[linear]

POOL-1670 TwabLib: _computeNextTwab should be casted to uint224

Line: https://github.com/pooltogether/pooltogether-contract-tsunami/blob/ec1e547eb5027b6c5323be8f9dcb25d89ae9b59c/contracts/libraries/TwabLib.sol#L322

In the **_computeNextTwab **function, we cast amount to uint208 but in the ObservationLib we can see that amount in theObservation struct is casted touint224.

(https://github.com/pooltogether/pooltogether-contract-tsunami/blob/3e535ef0acccd270324d47c22aa0215773173c25/contracts/libraries/ObservationLib.sol#L25)

Fix:

``**toUint208 **should be replaced by **toUint224**

Also see if we can pass _currentBalance in uint224

[asselstine]

do we need to cast here? current balance is uint224 and currentTwab.amount is uint 224. I think we can skip the cast