does these things: 1. removes birds in Locations that have only 1 bird in them (printing a warning message while it does so) 2. does the vza rescaling process and returns the output that is needed for the vza_assign function. (basically the ouput of the mean and var rasters). Should return a-bar and b-bar too --- all of that in a big list. Then with that output I can pump it all into vza_assign for a particular (left-out) ref_bird or for an assign bird. I should return the fitted model for each one.
private_rescale(birds, pred, std)a data frame like "ref_birds" in isotope_posterior_probs after the isopredictions
have been attached to it.
The raster of isoscape predictions
The raster of isoscape standard deviations