ATRLIF neuron model

[jlubo]

This adds to the Lava process library a Leaky Integrate-and-Fire neuron with adaptive threshold and adaptive refractoriness (ATRLIF neuron). Both a floating- and a fixed-point CPU process model implementation are contained.

I also have a working NC process model, based on microcode from Lava-DL, which I kindly received from @bamsumit. For that, I'd like to know how to proceed to integrate the model into Lava-Loihi.

As a side note, the neuron model has already been integrated into Brian2Lava.

Pull request checklist

Your PR fulfills the following requirements:

• [x] Issue created that explains the change and why it's needed
• [x] Tests are part of the PR (for bug fixes / features)
• [ ] Docs reviewed and added / updated if needed (for bug fixes / features)
• [x] PR conforms to Coding Conventions
• [x] PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• [x] Lint (flakeheaven lint src/lava tests/) and (bandit -r src/lava/.) pass locally
• [x] Build tests (pytest) passes locally (except for tutorial02_processes.ipynb, for unknown reasons)

Pull request type

• [ ] Bugfix
• [x] 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?

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What is the new behavior?

• ATRLIF neurons can be used within Lava. Users can decide between floating- and fixed-point CPU implementation.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Supplemental information

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[jlubo]

@PhilippPlank @weidel-p thanks for reviewing. I've pushed the adjustments you asked for, so I guess it can be merged now.

Regarding the NC process model, do you want to integrate this into Lava-Loihi? How can we proceed for that?

[PhilippPlank]

@jlubo Could you please update the branch with main? I had to fix a CI problem and this is now blocking this PR. Thanks!