Enable cost tracking with Loihi backend (#212)

[AlessandroPierro]

Issue Number: Closes #185

Objective of pull request: enable cost tracking for OptimizationSolver on Loihi backkend

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 current behavior?

• Cost tracking not enabled for Neurocore backend

What is the new behavior?

• Cost tracking enabled for Neurocore backend through the probe_cost variable of SolverConfig

Does this introduce a breaking change?

• No