Harmonize PSO and NM optimizers

[twallema]

Pull request checklist

• I have checked to ensure there aren't other open Pull Requests for the same update/change
• I have verified all tests work / have added new tests.
• I have verified all tutorials work.
• I have updated the documentation accordingly.

Describe your pull request

• Removed particle_output option from pso.
• Both PSO and NM now output "theta" (list; optimized parameter values) and "score" (float; corresponding objective function score).
• Avoids having to call pso/nm.optimize() awkwardly like:

theta = pso.optimize(...)[0]

• Replace by:

theta, _ = pso.optimize(...)

• I have considered only outputting theta to make this even "cleaner" but idk, sometimes having the score might be important.