Make leaf-node dispatch extensible

[SamuelBrand1]

This PR removes the pattern:

if node_type in [
            config.Constant,
            config.Linear,
            config.SquaredExponential,
            config.GammaExponential,
            config.Periodic,
            ]

which hard-codes the only possible kernels to have as leaf nodes, and replaces with

if NodeType <: GP.LeafNode

Which allows a user to define new <: LeafNode primitive kernels that can be recognised by the covariance_prior generative function.

test/test_extensible_config.jl gives an example of extending GPConfig defaults to give non zero probability of proposing a new leaf kernel with categorical(node_dist).

This PR unblocks #45

[fsaad]

Thank you for the PR, it will be reviewed and integrated for the next release.