Remove liquidity funcs and tests

[Tcintra]

Description

Addresses #258 (but only for StableSwap pools)

Added remove_liquidity and remove_liquidity_imbalanced for the vanilla StableSwap pools, along with their Vyper fixtures and unit tests. The unit tests just do the same as the other add/remove liquidity tests: it checks that the Vyper and Python instances of the 3pool will end with the same balances, tokens, and D.

The Vyper implementation is largely copied from what already exists in metapool.vy, with minor changes. The Python implementation is similar to the calc_token_amount function.

Disclaimer: this is my first time contributing and it is very possible that I am missing something. Please let me know and I will address any concerns.

Hygiene checklist

• Changelog entry
• Everything public has a Numpy-style docstring
  (modules, public functions, classes, and public methods)
• Commit history is cleaned-up with minor changes squashed together
  and descriptive commit messages following Tim Pope's style

Cute Animal Picture

[github-actions]

Coverage report

The coverage rate went from 75.75% to 75.96% ⬆️
The branch rate is 62%.

97.22% of new lines are covered.

Diff Coverage details (click to unfold)

curvesim/pool/stableswap/pool.py

97.22% of new lines are covered (96.79% of the complete file).
Missing lines: 554

[allt0ld]

Nice work (haven't reviewed thoroughly though). By the way, you can run make changelog_entry in your terminal, which will generate a changelog file. I would put your additions to pool.py and basepool.vy under the "Added" section in the changelog file. You don't need to describe the tests. Delete the other changelog file sections. Then, git add <changelog file path> so you can commit and push it.

Happy first contribution!

[allt0ld]

Good work @Tcintra. Before you merge, could you leave a comment in this thread explaining your use case for returning fees/amounts from remove_liquidity_imbalance in the .py version?

[Tcintra]

The remove_liquidity_imbalance function is used by crvUSD's PegKeepers when withdrawing crvUSD from StableSwap pools. We are emulating this behavior in our Agent Based simulation models for stress testing crvUSD.

Since curvesim's StableSwap pools don't track the individual balances of users, we need a way to track the balance of the PegKeeper when it mints or burns LP tokens. The add_liquidity functions returns the LP tokens minted, and we need the remove_liquidity_imbalance function to return the tokens burnt. Returning the fees paid is not strictly necessary right now, but might be useful for analysis on PK profitability in the future.