Update pcr.py
Update pcr.py

1. Added properly working method .X_scores_predict
2. Added properly working method .Y_predict
2. Removed cross-validation type "lpo" and implemented "lolo"
3. Adjusted computation of validated explained variance
4. Added mechanism for handling columns with zero variance when initialising NIPALS algorithm
5. Points 1, 2 and 3 were contributed by Kristian Hovde Liland
olivertomic/hoggorm