Added split gaussian option to fac!

[contagon]

Adds a from_split_cov and from_split_sigma methods to GaussianNoise. Essentially these are helpers specifically for creating noises for SE2/SE3 where the scales in the variable are inherently different.

Creating noises for these previously (and in other libraries) is tedious due to always having to repeat values. Instead, GaussianNoise::from_scalar_sigmas(0.1, 0.1, 0.1, 1.0, 1.0, 1.0) -> GaussianNoise::from_split_sigmas(0.1, 1.0)

Along with this, a new option to fac! is added,

assign_symbols!(X: SE3);
let factor = fac![prior, X(0), (0.1, 1.0) as std];