Add support for PersLayer

[marcusblake]

Import PersLayer to PyTorch topological. As apart of this effort, it would be nice to include the following deliverables:

• PyTorch layer
• Unit tests
• Example of usage
• Simple documentation on how this is used and why someone would find this layer useful to integrate into their network architecture

Useful resources for completing this task:

• PersLayer paper.
• Current PersLayer implementation in TensorFlow which is used by Gudhi.