LIF refractory floating point

[ssgier]

Issue Number: #282

Objective of pull request: Add LIF process with refractory period

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?

• No LIF process with refractory period is available

What is the new behavior?

• LIF process with refractory period is available. CPU/Floating point process model that implements the process is available.

Does this introduce a breaking change?

• Yes
• No

Supplemental information

As discussed with @mathisrichter, I created a new process based on the LIF process with reset. This change currently only includes the floating point process model in Python as a start. It could serve as a starting point to discuss/implement other process models.

[mathisrichter]

Oh boy, sorry @ssgier for the long delay. I have not been monitoring Github for a while and we overlooked this PR.
I have now assigned a few people from my team to review it.

Hopefully, we can review and get this merged soon now.
My apologies!

[weidel-p]

Thanks for this clean implementation there is not much to ask for.

The only thing is, that I would like to change the default value to 0 instead of 1. In your current implementation, 1 means "no refractoryness" and yields the same results as the normal LIF. In my understanding, 1 should mean that the voltage is clamped to zero for one timestep after spiking. Hence I would change the line

non_refractory = self.refractory_period_end <= self.time_step

to

non_refractory = self.refractory_period_end < self.time_step

and allow refractory_period to be 0 but not negative.

[PhilippPlank]

Looks good. Some minor comments from my side ;)