add regressor interface
add a label_preprocessing method
add 1st preprocess method (encode label)
Merge branch 'lihang00-master' into develop
issue: implement OWLQN and LBFGS and pass the unittests

issue: rename lbfgs_minimizer_test.go to minimizer_test.go
issue: add comments about owlqn minimizer

issue: disable the output of the minimizers
issue: add lr_owlqn and add it to the classifier test
enable regression

gaussian process for regression

gp is working, add test