get_trip_speed: Consider distance between first and last stop

[rafapereirabr]

In the get_trip_speed() function, the code currently considers the lenght of the entire trip (see below). However, it's quite common that the trip geometry stretches longer the the 1st and last stops. To get more accurate results, we would need to tweak the code to consider only the distance AND duration between the1st and last stops.

  # generate desired geometries - checking for required files/fields is done
  # pretty early into get_trip_geometry code

  trips_geometries <- get_trip_geometry(gtfs, trip_id, file)

  # calculate the length of each geometry

  trips_length <- sf::st_length(trips_geometries)
  if (unit == "km/h") trips_length <- units::set_units(trips_length, "km")
  trips_length <- as.numeric(trips_length)

[dhersz]

Related to #32.