Add selectors (FilterSelector and MuSelector) which determines how many solutions to use as evolution parents

[itsdawei]

Description

This PR is a split from #207

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

• MuSelector
• FilterSelector
• Documentation
• Testing

Questions

• Clarify how MuSelector is supposed to work

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

[itsdawei]

It is unclear whether a separate class is needed for each selector, as the current selectors (FilterSelector and MuSelector) are very simple. Until more future developments that justifies the use of a separate class, we will be closing this PR for now.