OPT: Clean up optimization API

[ntolley]

Several comments have been raised in #652 that can be broken into separate PR's:

• [ ] Add deprecation warning for optimized_evoked() import path (changes the default location of the function)
• [ ] Option to average over multiple trials when fitting to specific waveform
• [x] Separate classes or __init__() parameter when specifying Bayesian or COBYLA optimization
• [ ] Remove (or limit) Optimizer class parameters that are objective function-specific (e.g., scale_factor and smooth_window_len)