Multiband support for Raster zonal stats

[butchtm]

Right now, the create_raster_zonal_stats method of the raster zonal stats module only support single bands per call. Being able to handle multiple bands might be a nice enhancement to minimize having to do multiple passes on the same raster file across the same zones.
The underlying rasterstats modules has some suggestions on how to handle this (which I'm documenting below so that it may serve as a future reference for an implementation)

• Parallelizing the processing across different zones - ref issue
• Rasterstats multiband support discussion
• A recommended rasterstats Multiband implementation