[ENH] experiments: uniform call signature and terminology

[fkiraly]

In BaseExperiment descendants, makes the following consistent:

• __call__ signature is now __call__(params), instead of __call__(**params) - for consistency with evaluate and the expected function/callable interface
• the second return of score and evaluate is now consistently called "metadata" (instead of add_info), as referred to in the docstrings

Minor additional changes:

• removed some unused imports