Add Kernel Ridge Regression

[muammar]

I already did this before. I have to add this model and modify the data class so that the code is not slow. This is related to issue #2.

• Build kernel matrix per atom.
• Train model with Cholesky.
• Compute just triangular kernel matrix to save computational time.
• Logging different parts of the tasks.
• Save/load parameters to file.
• Predictions.
• Clean up the code.
• Parallelization.
• Make it possible to do a calc.get_potential_energy(atoms) in memory.
• Support sigmaper atom.