Dev/learning 2f on chip

[weidel-p]

Issue Number:

Objective of pull request:

This PR makes some minor changes to the Learning architecture to enable 2F on-chip learning for Lava-Loihi. Most notably:

• Introduction of the LearningLIF Process
• Introduction of the InAxonAddress for Loihi to enable get/set of pre-traces

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 (flakeheaven lint src/lava tests/) and (bandit -r src/lava/.) pass locally
• Build tests (pytest) passes locally

Pull request type

Please check your PR type:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
• Other (please describe):

What is the current behavior?

What is the new behavior?

Does this introduce a breaking change?

• Yes
• No

Supplemental information

[bala-git9]

Minor aesthetic changes that needs to done, but will still approve. Overall looks good.

[gkarray]

Looks good!