Fixed several things to do with fillers:
 * Checked the input value from cmd line for filler_dist properly. Checked it
   was between a range of 0 and a max distance.
 * Improved the logging when doing filler calcs, such as reporting existing
   stops found
 * Improved the algorithm to deal with a rare case of routes with only 1
   existing stop (e.g. looped routes, with a start/end).