[Enhancement]: Generalize approximate_kernel_expectation to non-square matrices

[Task]: Gamma node rules documented in Overleaf

[Enhancement]: rule derivations online

[Epic]: message passing rule documentation

`Gamma` node documentation and tests are low quality

Deprecated extension?

[Task]: MvNormalWishart node to enable joint μ–Λ priors for NormalMixture

Missing Average Energy for Mixture node

Different numeric types for the GammaShapeLikelihood constructor?

Incorrectly named tests cause some test files to be ignored