arcpy independence

[Jwely]

This is a big one, that must be slowly worked towards. Nearly all of what dnppy does can be done without using arcpy, but so much of our code was built with arcpy from the start. Now that gdal is easily set up with installation of dnppy, effort should be made to keep future functions free of the arcpy module. Nearly all of the arcpy dependent functions are within the raster and landsat modules.