Add hyperparameter optimizations for other covariance functions

We're going to support 4 covariance models:
* Exponential ( Matern with nu = 1/2 )
* Matern with nu = 3/2
* Matern with nu = 5/2
* Squared exponential
Fixes to covariance functions
Add to paper description of hyperparameter optimization
Merge branch 'master' of github.com:sdewet/gaussian_regression_project