Simplify getting started

[cthoyt]

Hello! I was just trying to run chebifier myself and thought there were a few ways it could be improved for quality of life.

This PR does the following:

1. Updates the constructor of BaseEnsemble
   • Make it possible to not have to pass a configuration. In this case, load the default configuration. Users highly appreciate there being a good default.
   • Make it possible to pass a file path, reducing verbosity of the example
2. Expose the BaseEnsemble from the top-level so it's easier to import
3. Simplify the example in the README by using the built-in configuration

[sfluegel05]

Hi! Thanks for your contribution. I think being able to use the BaseEnsemble class (or any ensemble class) without parameters would indeed improve usability.

I had some trouble running the code, in particular

1. Exposing BaseEnsemble to the top level lead to a circular import error:
   ImportError: cannot import name 'modelwise_smiles_lru_cache' from partially initialized module 'chebifier' (most likely due to a circular import) (python-chebifier\chebifier\__init__.py)
   I haven't pinned down exactly where this comes from and removed the __all__ statement for now.

2. The BaseEnsemble init can load the config, but there is some post-processing attached that so far only happened in the CLI. I have moved the post-processing to the utils and called it from the BaseEnsemble.

@cthoyt Could you check if this is what you had in mind?

[cthoyt]

@sfluegel05 thanks for taking a look! i'll try and resolve this ASAP and ping you here after

[cthoyt]

@sfluegel05 just resolved the circular imports

[sfluegel05]

thanks for fixing this!