Refactored curvature calculation into its own function for reusability.
#1 Splitting roads into smaller sections

Currently hard-coded to 1.5 miles (2400 meters)
No longer maintaining way refs after they have been converted to segments

This saves about 4MB of memory (from 470MB to 466MB) when processing Vermont.
#1 Curvature has learned the --straight_segment_split_threshold argument

This allows the user to pass a threshold in miles over which straight sections
will cause the way to be split. The default of 1.5 miles seems to
work well in Vermont. Setting to 0 prevents ways from being split.
Merge branch '1-Split_roads_into_smaller_segments'

This fixes #1.