Add conditional mean and cov

[SamDuffield]

Adds convenience functions to grab the analytical solution to the Fokker-Planck equation for OU processes

[SamDuffield]

The log_prob MLE test was failing on the CI (not exactly sure why), I took the chance to generalise to learning an asymmetric A and now it seems to pass. We should still think about how to speed this up though #32

[KaelanDt]

Looks good to me, but am a bit unclear on what the added code is/will be used for

[KaelanDt]

Suggested change

	"""Mean of p(x_t | x_0)
	"""Computes the mean of p(x_t | x_0)

[KaelanDt]

same applies in other places

[SamDuffield]

Looks good to me, but am a bit unclear on what the added code is/will be used for

I think these functions are useful to have and could find a variety of applications. Two concrete ones:

• If you want to compute e.g. KL or Wasserstein from another Gaussian distribution.
• Used as the dynamics of a Kalman filter and composition with Kalman prediction + update procedures.