MAINT: Remove unused and deprecated ConstantNumeric node class
BUG: Fix alpha beta recursion to not modify its inputs
DEMO: Add monitor option to drifting LSSM demo models

This option allows the user to choose whether the approximate VB
distributions are plotted during VB learning or not.
MAINT: Remove unused method Constant.compute_fixed_moments
MAINT: Improve error message in alpha-beta recursion
ENH: Implement Dirichlet message/statistics
ENH: Add delete method to nodes.

The method makes it possible to delete a whole branch from a model, that
is, a node and all its child nodes recursively. Maybe not very useful
for the end user, but the main usage could be the following: When
converting nodes to proper types just so that it is simpler to use, the
conversion node does not disappear but remains as a child of the
original node. Thus, there can be several "haunting" unused conversion
nodes. So, if the conversion node is not really used anymore, delete it.
ENH: Minor fixes to Mixture node
BUG: Fix ensureparents to remove unused temporary conversion nodes
ENH: Change the default rtol for assertAllClose in TestCase

Unit tests fails some times because of too strict rtol.
ENH: Use ensureparents in constructors

The decorator ensureparents also deletes unused temporary conversion
nodes if such are created.
ENH: Add CategoricalMarkovChain to nodes imported by default
ENH: Implement conversion from categorical Markov chain to categorical
TST: Add unit tests for categorical Markov chain