Include all layers in `custom_keras_layers` (#1280)
Include all layers in `custom_keras_layers` (#1280)

This adds all of the layers defined by StellarGraph to the `custom_keras_layers`
dictionary, which can be used for deserialising models created with
StellarGraph.

It does so by listing the layers in the source and iterating over them to create
a dictionary, rather than:

- trying to automatically get a list of layers defined by StellarGraph, because
  that seems difficult, and it would be easy for it to be silently and
  non-obviously adding extra (see discussion of CI below) or missing some
- defining the dictionary literally as done previously, because there's a risk
  of typos in the class names, which is eliminated by accessing the class's
  `__name__` property

I tried to add a test, but Python's `inspect` module gets confused on CI and
thinks that various Tensorflow classes are defined by Stellargraph, e.g.:

- test file:
  https://github.com/stellargraph/stellargraph/blob/d6ecf2c1d3a8ce048d7bc1f17c4ca569bd843a65/tests/test_init.py
- test results:
  https://buildkite.com/stellar/stellargraph-public/builds/3866#d3e4f9c2-4bf6-4ec0-84c8-9ae9ca565d84/299-384

In the interests of having this code landed, this doesn't include a test.

See: #1262

stellargraph/stellargraph,stellargraph/stellargraph