[FEATURE] build linear Gaussian chain rule utility function

[dehann]

A variable elimination game calls for calculating a new conditional from other factors (akin to "smart factors").

[dehann]

cc @Affie

[dehann]

From Kaess et al., "The Bayes Tree: An Algorithmic Foundation for
Probabilistic Robot Mapping", 2010

[dehann]

likely useful to #464

[dehann]

https://en.wikipedia.org/wiki/Schur_complement

[dehann]