Add support for the magnetic laplacian

[pulquero]

The magnetic laplacian provides an alternative to symmetrizing the adjacency matrix for directed graphs. See e.g. "Graph Signal Processing for Directed Graphs based on the Hermitian Laplacian".

[nperraud]

Could you maybe point us to a reference paper?

[pulquero]

The magnetic Laplacian is widely mentioned, my fav is https://ecmlpkdd2019.org/downloads/paper/499.pdf. Note for q=0, the magnetic Laplacian is the same as the symmetrised one, so you could just introduce q as a kwarg and default it to 0 to be backwards compatible.

[pulquero]

Btw, I'm not sure if it is intentional or not, but the directed symmetrised laplacian as it is currently in the code isnt using dw from the symmetrised W but self.dw.

[pulquero]

I've put together a nice demo here: https://github.com/pulquero/sgw/blob/main/examples/graphwave.py

[nperraud]

Thanks a lot. I need to have a look at what you did, but I am really overworked. I will try to do it asap.

[pulquero]

Came across another useful hermitian Laplacian for directed graphs:
https://core.ac.uk/download/pdf/206608976.pdf (Laplacians and the Cheeger inequality
for directed graphs, Fan Chung).