Started working on cross-validation tools.

Created TrainTestSplit function to split train and test datasets.
Refactored function to use mat64.

Changed most of the actual matrix operations to use the mat64 library.
Added Mac and IntelliJ files to gitignore.
Moved to own package and added concurrency support

Moved the function to the cross_validation package. Also modified the
shuffling function to run concurrently.
Merge pull request #15 from nbsantos/cross-validation

Cross validation