Add docstrings for Z_mean and Z_var
Variance should be sigma^2, not sigma
Update derivation notebooks for mean and variance
Update references
Update tests a bit
Update derivation for expected variance
Initial implementation of expected_Z_var
Add approximation functions
Update tests with approximation functions
Update visual tests with approximation functions
Fix naming
Rerun notebooks
Reimplement expected variance
Revert "Reimplement expected variance"

This reverts commit fcd86f11d9a989776ae686be65a337622b8259c2.
Misc bugfixes
Rerun example notebook
Rerun visual tests notebook
Fix improve covariance conditioning
Fix tests
Rerun example notebook