Add graph layers using spektral. Closes #213

[brollb]

This adds support for graph layers as shown below:

Remaining things before this can be merged:

• ~[ ] ensure the docstrings are rendered nicely (slightly different formatting for some of the new layers)~
• [x] more intelligent categories. All the new graph layers should probably just be in a category called "Graph" or something.
• [x] detect abstract layers more appropriately (eg, MessagePassing layer is abstract, I believe)
• [x] update keras seed