Implementing Manifold Learning and Reduction techniques

[ICGonnella]

New Features to add

• [ ] KernelPCABlock implementing the kernel PCApaper link
• [ ] MultiDimensionalScalingBlock implementing MDSpaper link
• [ ] IsomapBlock implementing Isomappaper link
• [ ] SpectralClusteringBlock implementing SCpaper link
• [ ] LocalLinearEmbeddingBlock implementing LLEpaper link
• [ ] DiffusionMapBlock implementing DMpaper link

How Kernel-PCA (kPCA) is the base class, all the others can be implemented as particular cases of kPCA with ad-hoc kernel functions