Starting an ARGS_SPEC for the decay function.

The decay function parameter is described as a non-user-facing parameter
that may need to be experimented with by the science teams.  I interpret
this as that it should therefore be exposed until further notice.
RE:natcap#722

src/natcap/invest/urban_nature_access.py

@@ -97,6 +97,20 @@
             'units': u.m,
             'expression': "value > 0",
             'about': "",  # TODO, will know more about this when I implement.
+        },
+        'decay_function': {
+            'name': 'decay function',
+            'type': 'option_string',
+            'required': False,
+            'options': [
+                'dichotomy',
+                # "exponential" is more consistent with other InVEST models'
+                # terminology.  "Power function" is used in the design doc.
+                'exponential',
+                'gaussian',
+                'density',
+            ],
+            'about': '',  # TODO
         }
     }
 }
@@ -198,6 +212,8 @@ def execute(args):
         target_path_list=[file_registry['aligned_population']],
         task_name='Resample population to LULC resolution')
 
+
+
     graph.close()
     graph.join()
     LOGGER.info('Finished Urban Nature Access Model')