Handing large polygons intersecting high-resolution rasters

[crvernon]

Development addressed the need to handle memory and processing time constraints that prevented large polygons from being able to fully extract grid cell values from a high-resolution raster. The following was conducted to deal with the issue:

• Large polygons for New Orleans | LA and Saint Louis | MO are now split into equal parts (the default is 40 though when this could likely be reduced depending upon the memory available in the machine processing the data)
• Each equal part is then used to extract the required information from the source raster in parallel (number of cores - 1).
• The outputs are then combined and returned.

This PR fixes #37

[KristianNelson]

Ran the larger watersheds and the smaller watersheds. Everything's working smoothly now.