Adding support for measurements in nautical miles, Issue #4017

I'm really new in this of contributing. I've just tocuhed some files to add support for nautical miles as I asked for in issue 4017.

I've checked measurments with a reference layer (one polygon and some lines) against PostGIS st_area measurements and ArcGIS and results are satisfactory.

I hope I've done all steps correctly, please tell me if not.

Thank you very much for all.