#49 Allow Routes and TrackOffsets to contribute to probability distribution.

[Paul J. Morris]

Given populated values for track offsets (and perhaps road/linear/drainage find locations), allow routes drawn on the map (and perhaps linear features in GIS vector base layers) to modify the probability distribution.

This issue and issue [#47] probably both entail a separate probability distribution layer, contributed to by various sources including the statistical circles layer, clues, dispersion angle, routes, and track offsets.