ENH: Remove default node statistics and remove as_gaussian method

Usually, the user should not worry about node conversions. Thus,
GaussianMarkovChain.as_gaussian was removed and one can use the general
_convert method if needed.

Node statistics default to None, so sub-classes must specify the
statistics object.
DEMO: Update the LSSM demos
ENH: Initial sketching of categorical Markov chains and Distribution