Large refactoring of the package.

[theogf]

Do the following changes :

• Externalize InducingPoints on a different package
• Change the data structure via DataContainer and parametrize the AbstractGP model on it.
• Rename all the Abstract_GP types into AbstractLatent types.
• Add a GPPrior and TPrior containing the kernel, the mean prior and the kernel matrix
• Add a LatentPosterior type containing the posterior parameters and eventualy more.
• Replace most getproperty functions by specialized functions, which helps removing some useless fields like nLatent.