TODO: New probeye design

[danielandresarcones]

New probeye features and modifications for easier development and implementations.

• Simplify the likelihood and model error.
• Restructure GaussianLikelihood and Correlation
• Custom Prior definition
• Check modularization

[joergfunger]

I would make a separate issue for the prior, the rest should be related to minimal changes such that we can get the current paper on the embedding into probeye. What would be the interface for the embedding (how do we define the embedded parameters, in particular when we start working on correlations)?