Attempt to enable regression, and first iteration of Gaussian Process

Added necessary supports for regression - please see if it is an OK way to do it.
A minimal version of GP is added to be a demo regressor.
A test (regressor_test.go) is added to verify it works. I have a graph of regression result in gp_test project of my github account.
On many datasets GP perhaps doesn't work yet because some hyperparameters are hard-coded.

TODOs for GP:
1. Allow passing in initial hyperparameters from command line options. SGD for hyperparameters.
2. More options for covariance functions.