Update the fee applied insimulate_swap and simulate_swap for UniswapV2Pool

[0xKitsune]

Right now, swap simulation uses a .3% fee, where not all Dexs that are a uniswap v2 variant have the same fee. Update swap logic so that the correct fee is used. Also, update Pool creation as well as Dex creation so that the fee can be dynamic (right now it is fixed to the .3% fee for these variants).