Prevent Optimizer from taking in non-unique emitter instances

[btjanaka]

Description

Users were previously able to do something like:

emitters = [GaussianEmitter(...)] * 5
optimizer = Optimizer(archive, emitters)

However, this is incorrect because emitters now contains 5 of the same instance of GaussianEmitter, when it should really contain 5 unique instances, which one would get if they instead did:

emitters = [GaussianEmitter(...) for _ in range(5)]

This PR makes sure Optimizer cannot take in an emitters list where any of the members have the same id().

Changelog Description

Fixed a bug where Optimizer is able to take in non-unique emitter instances

TODO

• Add test for this bug
• Fix the bug
• Add warning in documentation

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