Make EmitterBase easier to extend

[btjanaka]

Description

A clean-up of EmitterBase and its children to make it as easy as possible to extend.

Resolves #98

Changelog Description

• Backwards-incompatible: Reduce attributes and parameters in EmitterBase to make it easier to extend (Make EmitterBase easier to extend #101)
• In Optimizer, support emitters that return any number of solutions in ask() (Make EmitterBase easier to extend #101)

TODO

• Remove "children-only" / "protected" variables
  • Remove the _rng attribute and put it in the children, as not all emitters may need it
  • Make _archive a public attribute
• Remove the seed parameter (without _rng, there is no need for seed)
• Check that existing emitters are not accessing private attributes such as _lower_bounds and _upper_bounds
• solution_dim and bounds should be okay to keep
• remove batch_size
• make Optimizer keep track of the number of solutions generated
• The default tell() makes sense, but its docstring should be updated to be more clear on what it does
• Fix all existing emitters to use the new API
• Fix tests to not depend on old API

Questions

Status

• I have read the guidelines in
  CONTRIBUTING.md.
• I have formatted my code using yapf
• I have tested my code by running pytest (tox is run in CI/CD)
• I have linted my code with pylint
• This PR is ready to go