Update polygon_distance.py

This python script replaces the “Conefor Inputs”, a custom-made GIS extension for ESRI ArcGIS developed by Jeff Jenness Enterprises (www.jennessent.com) specifically developped for the Conefor software (CS, www.conefor.org).

From a polygon shapefile create in the output folder two txt files:
- Nodes: list of the IDs and areas of the polygons
- Distances: minimum Euclidean distance between every pair of polygons

These txt files are requested by Conefor to compute connectivity indices.