Need a better common base class for meta steps (that handles get_hyperparams and set_hyperparams)

[guillaume-chevalier]

Problem:

• MetaStep and MetaSteps doesn't implement get_hyperparams nor set_hyperparams.
• TruncableSteps does.

What should be done about it:

• Move some logic from TruncableSteps to MetaSteps
• Have MetaStep act the same, probably by inheriting MetaSteps but setting only one such meta step.

Other better solutions could probably be possible. Basically, we need not only to have nested (recursive) pipelines to be able to return their hyperparams, but also nested objects that are MetaStep(s). MetaStep(s) do contain other step(s) and should be able to get and set their hyperparams recursively as done in TruncableSteps.

@alexbrillant I'd like your thoughts on this.

[guillaume-chevalier]

Same goes for spaces with set_hyperparams_space.

[alexbrillant]

can we discuss this ? @guillaume-chevalier