make it easy to extract current parameters

[ExpandingMan]

Motivation and description

Currently it's pretty difficult to extract current parameters from a scheduler for two reasons:

• You typically have to descend into a parameter tree structure to even extract the scheduler state object in the first place. This is quite a pain to do from scratch, even with functors.
• There is no public method for getting the scheduler state.

The latter problem I think is fixed relatively easily: the currently private _get_opt should be made public. No obvious reason jumped out at my why this couldn't be done, though I'm not all that familiar with the code base.

The former problem I guess should be solved in Optimisers instead since it has Functors as a direct dependency while this does not. I suppose this would be done on the assumption that the schedules for all the leaf nodes are the same, otherwise it would be pretty inefficient.

Possible Implementation

Change _get_opt to public getopt (or whatever). Extracting from tree will have to be worked out more carefully.