DOC: Update documentation
DEMO: Major refactorization of LSSM demos

Cleaned and re-organized the code in several LSSM demos. The drifting
LSSM demo 2 was deleted. The drifting demo 1 is now called demo_lssm_tvd
where tvd stands for time-varying dynamics. Each of the demos demo_lssm,
demo_lssm_tvd and demo_lssm_sd have clear structure and they have
functions that can be used out-of-the-box for other datasets.