====== kalman filters ======
  * https://m.youtube.com/watch?feature=youtu.be&v=biY7F-tLwE8
  * https://www.bzarg.com/p/how-a-kalman-filter-works-in-pictures/
  * https://www.r-bloggers.com/kalman-filter-as-a-form-of-bayesian-updating/