Refactor require to check for position unit during engage (#29)

Author: Sachin

contracts/extensions/DeltaNeutralBasisTradingStrategyExtension.sol

@@ -1152,7 +1152,11 @@ contract DeltaNeutralBasisTradingStrategyExtension is BaseExtension {
     function _getAndValidateEngageInfo() internal view returns(LeverageInfo memory) {
         ActionInfo memory engageInfo = _createActionInfo();
 
-        require(engageInfo.accountInfo.collateralBalance == 0, "PerpV2 collateral balance must be 0");
+        // Check that there is zero position unit of USDC of collateral value. Allows to neglect dust amounts.
+        require(
+            engageInfo.accountInfo.collateralBalance.preciseDiv(engageInfo.setTotalSupply.toInt256()) == 0,
+            "PerpV2 collateral balance must be 0"
+        );
 
         return LeverageInfo({
             action: engageInfo,