[ENH] allowing old function API to be passed as experiment (#152)

From a user perspective, the ability to define ad-hoc functions via the
old `my_callable(opt)` notation was quite nice, and imo should be
retained for compabitility with user code, and due to quick
specification.

This PR adds

* the ability to pass functions of the old `my_callable(opt)` signature
to any optimizer
* a `FunctionExperiment` which adapts functions to the `BaseExperiment`
API. This experiment is public, since it is also useful when a user
already has a function or callable to adapt.

The feature also requires to relax the `BaseExperiment` contract, and
allow passing of `params` with unknown keys, or arbitrary keys, or keys
not known in advance.

Author: fkiraly