Fast distance

This pull request adds a method for calculating the closest edges, returing just the numerical value for the distance, and not the point coordinates. It is a lot faster, because it uses core QGIS and GEOS functions, implemented in C++.

The fast calculation cannot be used to generate the vector shapefiles with the distances, but it is helpful for the text file creation.

The pull request adds some other minor changes, as requested by user testing