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.

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