Recodes the -1 raster values used to denote the area within the study area, but outside the model habitat zone to NA. HSI models assign areas inside each model's habitat zone an HSI value of 0-1, while areas outside a model's habitat zone are assigned a value of -1 to distinguish it from areas outside the study area which are assigned the value NoData. Some model summary functions require that these placeholder values (-1) are converted to NA.