fix balancer stable pool calcs

• stable calc in get_amount_bpt_out
• stable calc in deposit_and_stake
• stable calc in deposit_and_stake_single_asset
• stable calc in unstake_all_and_withdraw_all
• stable calc in unstake_and_withdraw_all_single_asset

balancer eventually added this to their stable math docs...

lets track that message and move on when they have this fixed.

ref: https://dev.balancer.fi/resources/pool-math/stable-math

hmm i dont see the python warning anymore on that url 🤔

@jalbrekt85, could you add an update of the current state for this ticket?