Crop Production Percentile model: update number of supported crops from 175 to 172

[emilyanndavis]

While working on InVEST #614, I noticed the number of crops in the supported crop list for the Crop Production Percentile model is 172, not 175 as advertised in the User Guide.

Compared to the full list of 175 crops in EarthStat's Global Harvested Area and Yield for 175 Crops Metadata and Technical Documentation PDF, our list is missing coir, gums, and popcorn.

It seems appropriate to continue excluding these 3 crops from the model since, for whatever reason, their associated datasets are not available from EarthStat's Harvested Area and Yield for 175 Crops data download page .

Still, for the sake of precision, we should probably update 175 to 172 in the User Guide, and it might also be worth adding a short note about the exclusion of coir, gums, and popcorn due to unavailable datasets.