Recurrent netx save and load

[timcheck]

Issue Number: https://github.com/lava-nc/lava-dl/issues/327

Objective of pull request: As a user, I want to be able to save and load dense recurrent networks from lava-dl into lava.

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
• [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?

Cannot save/load recurrent network with NetX.

What is the new behavior?

A new hdf5 layer category for dense recurrent layers allows saving recurrent networks to an hdf5 file.

NetX can load into Lava a recurrent network from an hdf5 file.

A unit test shows example usage. An example 2-layer 5-neuron dense recurrent network is created in Lava-dl, saved to a hdf5 file, and loaded into Lava from the hdf5 file. The network is run in both Lava-dl and Lava on an example input to confirm matching activity level within an error margin of 2%.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

[timcheck]

Thanks @bamsumit, I converted the example to a unittest.