Make tuner and postprocessor dictionaries more friendly to users

## Background
Currently the models and their hyperparameter distributions are defined using dictionaries. While this makes sense to someone familiar with pyMAISE this may make attempting to understand how to build a new model confused. There may be a more object oriented approach here; however, I think this will touch a lot of the documentation.

## Tasks
- [ ] Update `pyMAISE/tuner.py` and `pyMAISE/postprocessor.py to use new input structure.
- [ ] Update all relevant docs in `docs/source/` including benchmarks.

## Definition of Done
The new input for users is more friendly to users (not dictionaries) and the documentation reflects the changes.