Add Support for Categorical Analysis with a Threshold

By adding a new type of analysis where a threshold is specified, I could start calculating which predictions were within that threshold from the real values, and start calculating things like heidke skill score (HSS), f1 score, precision, and recall. These metrics would be extremely useful to report, and HSS in particular seems to be one of the most popular units of measurement for comparing weather models across different resolutions, datasets, and spatial locations.