[Feature] Remove liquidity from a single asset

[MickdeGraaf]

User Story

• As an <audience/user>: User
• I want to <action/task>: Remove my liquidity in a single asset
• so that <outcome/goal/benefit>: So I don't have to liquidate all the unwanted assets

Type

• Enhancement
• Maintenance
• Refactor

Definition of Done

• function exitswapPoolAmountIn(uint poolAmountIn, address tokenOut)
• function exitswapExternAmountOut(address tokenOut, uint tokenAmountOut)
• Test coverage in unit test
• Tested in mainnet test

Attach files or screenshots if it's helpful for this issue.

Reference implementation: https://github.com/balancer-labs/smart-pools/blob/master/crp/contracts/ConfigurableRightsPool.sol

[dmvt]

https://forum.piedao.org/t/bounty-program-votes/118/2
PieDAO governance has voted to approve this issue for a bounty:
Approved Scope: 8 - 16 hours (2795.2 - 5590.4 $DOUGH)
Contributor: @travisdmathis

[travisdmathis]

Leaving these as done until I can be paid out for them

[MickdeGraaf]

Implemented in #51