Major API Changes for Emitter

[itsdawei]

Description

This PR introduces major API changes for the Emitter class. Emitter class now requires a Ranker object and a Selector object.

The Ranker object will define a rank function which returns the index of the solutions in the descending order that they should be in. It will also define a reset function which resets the internal states of the object.

RandomDirectionEmitter, OptimizingEmitter, and ImprovementEmitter are now all combined into EvolutionStrategyEmitter, which can be configured to use different Ranker's.

We also introduce FilterSelector and MuSelector objects which defines a select() function that decides how many parents to select from the potential parents.

EvolutionStrategyEmitter(x0, sigma0, ranker=Ranker, string, selector, ES, restart_rule, bounds, batch_size, seed)
If ranker=str, then we create the object for you

TODO

Rankers

• Implement EvolutionStrategyEmitter
• Implement ImprovementRanker
• Implement TwoStageImprovementRanker
• Implement RandomDirectionRanker
• Implement TwoStageRandomDirectionRanker
• Implement ObjectiveRanker
• Implement TwoStageObjectiveRanker
• Implement get_ranker(str) -> Ranker

Selector

• MuSelector
• FilterSelector

RestartChecker

• come up with an API for this

Others

• Add documentation for all new classes/functions
• Handle repeating docstrings using methods of https://github.com/mwaskom/seaborn/blob/9d8ce6ad4ab213994f0bc84d0c46869df7be0b49/seaborn/relational.py#L719

Status

• I have read the guidelines in CONTRIBUTING.md.
• I have formatted my code using yapf
• I have tested my code by running pytest
• I have linted my code with pylint
• I have added a one-line description of my change to the changelog in HISTORY.md.
• This PR is ready to go

[btjanaka]

Make the directory name be rankers

[btjanaka]

Let's make a restarter object just like for selectors and rankers.

[itsdawei]

Closing this PR because the changes intended by this PR has been dealt with in #209, #217, and #220.