Intro nb added
clean up and regression
preprocessor example
some more additions
revamp structure
some cleanups
more cleanups
update the lbfgs optimizer
update the unit tests for lbfgs
update krr example
Merge pull request #2244 from Saurabh7/intronb

Intro nb added
fix a bug
Merge pull request #2337 from yorkerlin/develop

update the lbfgs optimizer
Merge pull request #2343 from lambday/feature/selection

Added CPreprocessor::apply() and CFeatures::copy_dimension_subset() methods
Added feature selection
Minor changes in error msgs in DenseFeatures and SparsePreprocessor