ICML 2023: Initial commit of Simplicial Neural Networks

[Hellsegga]

This is a minimal first pull request for an implementation of Stefania Ebli, Michael Defferrard and Gard Spreemann. Simplicial Neural Networks. TDA {&} Beyond. 2020.

Comparison with the code of the authors of that paper plus extension or addition of new examples in the tutorial will be done later.