Update gSSURGO queries after apparent API changes

[infotroph]

I don't know what exactly changed or when in the gSSURGO API, but I was not able to make gSSURGO.Query or extract_soil_nc work in their current form -- this is my attempt to reverse-engineer a fix.

Input requested especially on my big deletion in extract_soil_nc, which completely drops the use of gdal in favor of asking the API to only return the mukeys in the first place. If there's a reason the gdal steps were necessary, please educate me!