fix SHN segments

[tiffanychu90]

shared_utils

• Follow up to TODO items listed in update shared_utils and segment_speed_utils / fix SHN segments #1283
• Try a different approach to cutting SHN postmile segments so that we don't have any segments that are missing
  • Instead of snapping it to the nearest coordinate within SHN linestring, we'll figure out where the postmile start and end falls relative to the SHN linestring
  • Subset the SHN linestring with postmile endpoints + SHN coordinates in between
  • No missing segments now, and it's cut as desired
• Research Request - Map PeMS data #1233