Updated documentation and functionality

Documentation Updates

• new vignettes (algorithms and co-prediction)
• updated existing vignette
• making sure vignettes are properly included

Code Updates

• misc. bug fixes (index checking)
• changing functionality of requesting model_output to be a list column (to be similar to functionality of Gaussian Process predictor)
• added Gaussian Process predictor
• s-map prediction variance computed based on neighbors similar to simplex
• only return predicted rows in model_output and smap_coefficients