Bug with masking

[ph-kev]

In make_lonlat_mask, the function should not preprocess the mask if set_to_val is not nothing. In the preprocessing, we should always make the mask into 0 and 1. We should then pass in true_val and false_val into the masking function that is returned. After we generate the appropriate mask (after rounding the values) with the same size as the data we are masking, we replace the 0 and 1 with true_val and false_val if needed. Finally, we return the new OutputVar.