TST: Add more unit tests for the moments conversion system

These unit tests check for the bugs that were just fixed.
ENH: Add ndim to GaussianGammaISOMoments
TST: Add unit tests for Gaussian-Wishart nodes
ENH: Refactor all Gaussian stuff into gaussian module

Gaussian, Gaussian-Wishart, Gaussian-gamma and similar nodes,
distributions and moments are all defined in gaussian module.
DEMO: Fix bayespy.plot API in demos
BUG: Fix axis index in add_axes function
ENH: Add error for no moments conversion cases
TST: Fix moment conversion unit tests for the new error class
BUG: Robust combination of parent moments in Mixture
API: Change GaussianARD API

GaussianARD now accepts GaussianGammaARD parents. Also, the shape of the
mean parent is interpreted as a concatenation of the plates and the
dims. This is simpler than before and allows one to use fully factorized
mean for variables that are not fully factorized (this works because the
ARD prior is fully factorized).
ENH: SumMultiply accepts GaussianGammaISO parents