Add marginal price capability and update integration tests

Balancer now passes the test
Some misc aesthetics

evm/src/balancer-v2/BalancerV2SwapAdapter.sol

@@ -188,11 +188,10 @@ contract BalancerV2SwapAdapter is ISwapAdapter {
         override
         returns (Capability[] memory capabilities)
     {
-        capabilities = new Capability[](4);
+        capabilities = new Capability[](3);
         capabilities[0] = Capability.SellOrder;
         capabilities[1] = Capability.BuyOrder;
         capabilities[2] = Capability.PriceFunction;
-        capabilities[3] = Capability.HardLimits;
     }
 
     function getTokens(bytes32 poolId)

evm/src/interfaces/ISwapAdapterTypes.sol

@@ -33,7 +33,11 @@ interface ISwapAdapterTypes {
         ScaledPrices,
         // Indicates that if we try to go over the sell limits, the pool will
         // revert (optional)
-        HardLimits
+        HardLimits,
+        // Indicates whether the pool's price function can be called with
+        // amountIn=0 to return the
+        // current price (optional)
+        MarginalPrice
     }
 
     /// @dev Representation used for rational numbers such as prices.