Fix: subthreshold dynamics equation of refractory lif

[monkin77]

Issue Number: #841

Objective of pull request: Fix the PyLifRefractoryModelFloat voltage dynamics.

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)
• [x] 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
• [] 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:

• [x] 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?

• The PyLifRefractoryModelFloat model does not update the voltage correctly in its sub-threshold dynamics. The current equation is: Therefore, the voltage of the neurons is updated incorrectly.

What is the new behavior?

• The correct dynamics of the voltage should be: so the new behavior uses this new equation.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Supplemental information

The unit test related to the Refractory model confirmed that the voltage remains at 0 during the refractory state. However, the expected voltage array is incorrect, so it was also modified.

Closes #841

[jlubo]

Hi, just to support this, in the Brian2Lava project we have used the same fix and it works well.

[monkin77]

@mgkwill @bamsumit I would like to request a review of this PR. It's a quick hotfix on the refractory LIF behavior.