Add "Hydrometeor Classification" display control for the NEXRAD dual pol product

A new "Hydrometeor Classification" display control for the level 3 NEXRAD dual-polarization product.  It's just an "Image Display", but the cursor readout functionality (middle mouse button) is overridden so that instead of "60", "70", "80", etc. you'll see "Light-Moderate Rain", "Heavy Rain", "Big Drops Rain", etc.

I was able to make the new display control only show up in the Field Selector for applicable data sources, but annoyingly it is the last choice in the list...

I also made the "Hydrometeor Class" the default color table when you display this product.

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