Bug fixes for setting seed and predicting from saved models

• Bug fix: save_model() output now works correctly with predict_spectra().
• Bug fix: train_spectra() no longer returns an error when stratified.sampling = F.
• In train_spectra(), stratified random sampling of training and test sets now allows the user to provide a seed value for set.seed(). For random (non-stratified) sampling of training and test sets, seed is set to the current iteration number.
• Minor documentation updates added.