ENH: Add mean function which ignores NaNs. Add unit tests for it.
BUG: Implement mask messages in Mixture. Add unit tests for it.
TST: Add a test for multiple axes in logsumexp
ENH: Ensure Gaussian statistics when plotting a Gaussian variable
BUG: Do not transform hyperparameters before they have been updated
DEMO: Add switching LSSM to drifting LSSM demo as an option
ENH: Improve switching LSSM demo