Handle pool balancing in factory function

[chanhosuh]

Remove D computation from the pool snapshot. It is only needed for creating balanced pools. This should be done explicitly in the factory function get_sim_pool. The regular get_pool doesn't appear to need this extra functionality so we can remove the balancing options from that.

Fixes #220.

Let's do this as 2 PRs to be safe:

• fix for stableswap first (check tests still pass)
• fix for cryptoswap