Excitatory/Inhibitory LIF neuron model

[monkin77]

Issue Number: #887

Objective of pull request: Model E/I LIF neurons to represent more complex behaviors containing positive and negative inputs, each with their own decay time constants (du_exc and du_inh). It also allows varying the voltage and current time constants of neurons belonging to the same layer.

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

• 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 new behavior?

• A new neuron model that allows playing with excitatory and inhibitory inputs and the variation between their underlying current time constants (du_exc and du_inh) to represent more complex scenarios

Does this introduce a breaking change?

• Yes
• No

Supplemental information

This only contains the floating-point precision version of the model. I believe it is possible to implement a fixed-precision version of it, although it requires further investigation.

I would appreciate your opinion on this as I've not delved deep into the microcode documentation.