Add Kernels!

[jhrudden]

Ticket Goals:

• - Add base kernel class for both Kernels composed by infinite (guassian, laplacian) and finite (polynomial) feature maps.
• - Add kernel PCA, LDA, Regression
• - Implement Random Fourier Feature for feature map approximation
• - Implement Nystrom For Kernel Approximation
• - Add notebooks for mean kernel embeddings
• - Take a crack at Kernel Density Estimation