Fixed rounding issue with recovery

[aalavandhan]

The computeRedeemableTrancheAmounts should round the balances down to a number divisible by the tranche ratio.

For example, if the tranche balances are "10, 15, 7.461048491123254231" the previous implementation returned "2.984419396449301692, 4.476629094673952538, 7.461048491123254231".

However, this is not the right redemption ratio, which is expected to be precisely divisible.
https://github.com/buttonwood-protocol/tranche/blob/main/contracts/BondController.sol#L229

The proposed change rounds down to achieve amounts are consistent with the expected tranche ratio.
We get -> "2.984419396449301600, 4.476629094673952400, 7.461048491123254000"

[aalavandhan]

Consider another example. This time lets just consider fixed point numbers.

For tranche ratios of 200,300,500 and granularity=1000.

Lets assume balances are "[1000, 5001, 503]"

As per the previous implementation, the redeemable tranche ratios returned will be "[201, 301, 503]".
The actual ratios are "[201.2, 301.8, 503]", but because we use fixed point math we lose the decimals and we get inconsistent ratios.

The expected solution in this case is simply ["200, 300, 500"].

The proposed solution first finds the closest balance divisible by the tranche ratios. (line 219) . We get "[1000, 4800, 500]". After this we just continue with the existing algorithm.

[brandoniles]

Nice solution. I agree this strikes the right balance for gas efficiency.