DEMO: Add "no-plot" switch to LSSM demo
DEMO: Add an SPDE demo for drifting LSSM
TST: Add moment checking for drifting LSSM
ENH: Improve the implementation of drifting LSSM
ENH: Add drifting LSSM to automatically loaded nodes
DEMO: Drfting LSSM demo 2 works

A second more complex demo for drifting linear state-space model works
nicely. The demo simulates a advection-diffusion stochastic partial
differential equation process with changing velocity field. Standard
LSSM does not perform as well as drifting LSSM.
ENH: Add ENV to gitignore in order to ignore virtual environment