Plateau Neuron - Fixed Point Model

[kds300]

Issue Number: #775

Objective of pull request: Add Plateau neuron model to lava as described in the above issue.

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

Pull request type

Please check your PR 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?

• There is no current behavior, this is a new model.

What is the new behavior?

• The model will take two separate inputs, one to the "soma" and another to the "dendrite". Both the soma and dendrite potentials will follow sub-threshold LIF dynamics. When the dendrite potential crosses its threshold it resets and sends the neuron to the UP state. The UP state will last for a specified duration of time steps. During this time, the soma is able to spike and reset if its potential crosses its threshold. If the neuron is in the DOWN state, the soma potential will follow subthreshold LIF dynamics regardless of whether it crosses the threshold.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Supplemental information