Develop YieldSpace-tv

[devtooligan]

What
Replace the plain ERC20 token used as base in Pool.sol for a Yearn Vault Token.

Why
Returns for liquidity providers are not competitive. Allowing the pools to keep Yearn Vault tokens as a replacement for the base will add significant ROI to liquidity providers.

[alcueca]

Includes:
#5 - Please check assets are not absorbed when selling.
#24 - The *Ratio parameters on init are useless.
#25 - The remainder parameter on init is useless.
#27 - Implement YieldMathExtensions