Implement evaluation of a Gaussian density in `utils`

[xEnVrE]

There are several places where we are evaluating a Gaussian density, namely

• GaussianLikelihood::likelihood
• WhiteNoiseAcceleration::getTransitionProbability
• GPFCorrection::evaluateProposal

and we will have other such as *KFCorrection::getLikelihood as per #42.

Can we have a utility function in utils.cpp for this? Of course, one day we may end up having a class for distributions.

[claudiofantacci]

Yes! Let's move this in utility for now, but I think we might need something more structured.
Maybe after we implement noise classes we can come up with a better solution.

[xEnVrE]

Ok, then let's say we can have a PR for adding this to utils and using it in all the places where we need it plus in *KFCorrection. Is this fine?

[claudiofantacci]

Sure. Are you willing to do it?

[xEnVrE]

Yes!

[claudiofantacci]

Assigned 🚀
As I'm also working on utils.h and is now a template header-only utility file. As a consequence, please implement everything in the header and possibly/if needed as a template, otherwise let's talk f2f.