Improved OptimizationSolver API

[AlessandroPierro]

Issue Number: #161

Objective of pull request: update OptimizationSolver API to return a complete report of execution

Pull request checklist

Your PR fulfills the following requirements:

• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
• PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• Lint (pyb) passes locally
• Build tests (pyb -E unit) or (python -m unittest) passes locally

Pull request type

Please check your PR type:

• Feature

What is the new behavior?

• The OptimizationSolver can be configured passing a SolverConfig containing all parameters related to the execution
• The OptimizationSolver.solve method now returns a SolverReport instance containing all the results from solver execution, profiling, and the parameters used for the exection (improving reproducibility).
• The solve method (available through from lava.lib.optimization.solvers.generic.solvers import solve) provides a simplified interface to obtain just the best solution passing a problem and, optionally, a SolverConfig instance.
• The SolverTuner interface has been updated accordingly, enabling the definition of arbitrary fitness function in the form f : SolverReport -> float.

Does this introduce a breaking change?

• Yes

[GaboFGuerra]

Thanks Alessandro, Please update the docstring of the fitness_fn before merging, it still has the old signature.