Support for FunctionTransformer

[janvanrijn]

I just came across the FunctionTransformer. Does the flow_to_sklearn / sklearn_to_flow support this module? If not, do we block uploading this?
(Just putting it out here, will write a MWE when I have time)

[amueller]

It seems to work. The function is pickled so it works as long as the function is available somewhere, just like any estimator.

I don't see a reason to block this. See https://openml.org/f/6554 for an example

[amueller]

@mfeurer suggests adding an example and a unit test.
Using np.exp works but lambda x: np.exp(x) probably doesn't work without having the function defined and named somewhere.