Usable Release

The Squared-Exponential kernel is not used by default in favour of greedier polynomials and periodic functions.
Sigmoidal and Change kernels now have a fixed slope (of 0.5) by default (instead of having it as an additional parameter to optimise), making them more competitive.
The linear kernel with horizontal offset has proven its worth against the one without it (better and more interpretable fits even considering the extra parameter).