AssetIndicesMixin asset_names setter and tests

[allt0ld]

Description

• AssetIndicesMixin children such as SimCurvePool and SimCurveMetaPool now have a convenient interface for setting the AssetIndicesMixin.asset_names attribute. Basic checks ensure that no duplicate names are set and that the number of asset names remains consistent after initialization.
• In AssetIndicesMixin.asset_balances, the instance raises a CurveSimException if its asset_names and _asset_balances aren't the same length, ensuring that they will be.
• Expanded test coverage in curvesim.test.unit.test_coin_indices_mixin. Invalid data such as nonexistent asset indices or duplicate asset indices are passed to AssetIndicesMixin.get_asset_indices() to check the proper errors are raised.

Resolves #172, resolves #187.

NOTE: should only be merged once metadata is set on CurvePool, CurveMetaPool, CurveCryptoPool __init__ rather than after, which is how it's currently done (#195). SimCurveCryptoPool will have to implement the setter after this gets merged.

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

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