BUG: Fix plates in computation of g in multinomial node

Make the fix and add unit tests. The bug left one axis that was
supposed to be removed from g.
BUG: Handle g properly in Mixture if it has too few plates

Mixture raised errors if g didn't have the cluster plate.
It is ok to broadcast it so this was fixed. If the cluster
plate was not present, Mixture doesn't try to move it but
instead create a new axis.
MAINT: Add some docstrings and clean the code a bit
MAINT: Remove a debugging message in Mixture
ENH: Add Hinton diagram plotter for Beta nodes
DOC: Add a sketch for discrete HMM example
ENH: Raise an error if creating model with non-node objects
DOC: Add discrete HMM section to examples