Enable default extraplotion of control input

Add related packages to README

Add reference for SR3 trimming

Update README.rst

Merge pull request dynamicslab#84 from dynamicslab/cross-validation

Cross-validation

Merge pull request dynamicslab#82 from dynamicslab/fix-check-fitted

Fix check fitted

Fix bug in score function

Bug fix: make check_is_fitted call compatible with sklearn v0.21

Define constant for coef_ thresholding

Remove extraneous print statement