Expose erosion strength as CLI argument

In the NAN project, the eps variable was very important to generate the data. We need a way to set its value through the CLI.

We can set 2 as default value, help="Strength of the erosion operation done on polygons"

Please update the Readme as well, with the following description:

The higher the value, the more the polygons will be eroded. A value higher than 2 is not recommanded as polygons might be removed as they become too small.