Improve `create_pool` by encapsulating optional parameters

[maltekliemann]

There are a lot of optional parameters required for creating Balancer-style pools. From a design point of view, this is a code smell. Either weights and swap_fee must both be Some(_), or both be None. Anything in between is questionable.

I suggest the following solution: Remove the ScoringRule enum from create_pool and instead use the following parameter:

enum<T> CreatePoolParameters {
    Cpmm(BalanceOf<T>, BalanceOf<T>, Vec<u128>, bool),
    Rikiddo,
}

This reduces the complexity of create_pool immensely:

fn create_pool(
    who: T::AccountId,
    assets: Vec<Asset<T::MarketId>>,
    base_asset: Asset<T::MarketId>,
    market_id: Self::MarketId,
    params: CreatePoolParameters,
) -> Result<PoolId, DispatchError> {

The create_pool function can then match the creation parameters accordingly.