Monte Carlo + Chi Square tests for OD

This greatly helps for the validation.

Take inspiration from gokalman:

• https://github.com/ChristopherRabotin/gokalman/blob/master/chisquare.go
• https://github.com/ChristopherRabotin/gokalman/blob/master/montecarlo.go
• https://github.com/ChristopherRabotin/gokalman/blob/master/montecarlo_test.go (includes the Chi squared tests)