ENH: Add a method for drawing random samples uniformly on a sphere
ENH: Add some map plotting utilities
ENH: Add a method for switching the plotter of a node
ENH: Move Haversine distance to utils and implement alpha-beta recursion

Haversine distance function is used to compute distances on a sphere.

Alpha-beta recursion computes the expectations and normalization
constant of a Markov chain. It takes as input the (unnormalized)
log-probabilities of the initial state and state transitions.
TST: Add unit tests for alpha-beta recursion
ENH: Add Statistics class (message type) for all nodes
DEMO: Make plotting of loading matrix in drifting LSSM demo optional