Consistent definition of sigma_f and sigma_n

[manal44]

In proFit, the hyperparameter vector used is: [ l=length-scale , sigma^2 = (sigma_n/sigma_f)^2 ] in order to normalize.

Adapt the written functions to this definition:

1. Replace l^2 by l in the functions' arguments.
2. Add a documentation for sigma .
3. Add an indication about the choice of sigma_f (ex: sigma_f always equal to 1) since it isn't a parameter of the kernel functions.
4. Handle the evantual different values for the same variable sigma_f given that it becomes an implicit argument for the functions which build the Covariance Matrices K(X_test,X_test) ; K(X_test,X_training) ; K(X_training,X_training) .

[krystophny]

Is this now consistenst @mkendler ?

[mkendler]

This is consistent, as we use sigma_f and sigma_n as explicit arguments now.