State space models with diffuse initial conditions

[fipelle]

Add a simple implementation to handle state space models with diffuse initial conditions.

[fipelle]

@azev77 I have implemented the so-called sequential / univariate approach to Kalman filtering and smoothing. This approach is used instead of the default filtering / smoothing by default if the covariance of the measurement errors is a UniformScaling{Float64}. I am planning to extend it to handle the exact diffuse initialisation in Durbin and Koopman (2000).