Started a new file for arcpy macros, and a few other changes.

Added a new file SFR_arcpy.py to hold general functions that combine   
   various arcpy commands like macros. Currently, only added a faster
   version of join called general_join that keeps only common fields
   and saves down the join. It's faster than JoinFeature_management.

Other changes on this commit are implementing the general_join in a few places
   and removing the compute_zonal function from SFR_preproc.py. It was not
   typically being used anyway.

   Also added a join into the Finalize script to bring in row and column into
   rvier_explode.shp from the model grid if it's missing.