Add NDVI, pseudo-NDSI, other options

[spestana]

Right now, the model train and predict functions expect only 4 bands from Planet Scope images. However, we want to allow a user to also create new "bands" to add to their model, such as NDVI or pseudo-NDSI derived from the Planet Scope bands, or even static land cover features (e.g. forest cover) or terrain maps (e.g. slopes, aspects, elevation). Therefore we should include functions to compute NDVI, pseudo-NDSI, or blue thresholding (Thaler et al., 2023), and functions to allow model train/predict with more than 4 features/bands.