Fix: invariant check in swap

[Alexander]

Description

In swap, the virtual reserves per liquidity are both rounded down:

portfolio/contracts/Portfolio.sol

Lines 475 to 476 in 7862676

 iteration.virtualX.divWadDown(iteration.liquidity), // Expects X per liquidity. 
 iteration.virtualY.divWadDown(iteration.liquidity), // Expects Y per liquidity. 

However, we are aware of needing to round in the proper direction, and thus we made changes in _getLatestInvariantAndPrice function to do that. To be consistent, we should further investigate if we need to make the same change here.

This should go along with fixing #339

[Alexander]

Fixed in #372