Extend pool factory function to cryptoswap

[chanhosuh]

Currently curvesim.pool.make only creates stableswap pools. This should be extended to enable the creation of cryptoswap pools.

Ideally the factory interface would be painless, e.g. "tvl" could be a parameter, instead of the user specifying D or xcp. In other words, the user creates an intuitive spec of a pool, and the factory understands the right kind of pool to create.

[chanhosuh]

On second thought, might be best just to have two separate functions for stableswap and cryptoswap