define a SWAT model object
set the model working directory. Default is '.'
set a subbasin shapefile object if output unit is subbasin
Example:
>>> [shp.subbasin] >>> source = 'GIS\subbasin.shp'
>>> [swat.watershedmodel] >>> shpsub = {shp='subbasin'}
set a HRU shapefile object if output unit is HRU