Steppable Optimizers

[MarcDrudis]

Summary

We incorporate a new subclass of Optimizer called SteppableOptimizer. A steppable optimizer has a method step() that allows to perform one single step in the optimization process as opposed to having to perform the whole minimization of the objective function.
This way the user has more options when dealing with complicated functions to minimize.
In this spirit, we have chosen to add ask() and tell() to this interface as well. More information about this can be found in the documentation.

The main goal of this PR is to implement the interface of the abstract class SteppableOptimizer and document it properly. In order to make some tests, we have implemented GradientDescent using this new framework.

Details and comments

A class CMA-ES is also in the making, but I prefered to start by making this PR with GradientDescent because it is easier to implement and later on we can add this other class.

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✅ MarcDrudis
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