Match expects the distance attribute to be a float

[sjirwin]

Match() defines its distance attribute as float and code elsewhere assumes that it is a float. e.g., from compute_cutting_points()

if abs(m.distance - distance_epsilon) < cutting_thresh:
                        cutting_points.append(CuttingPoint(i))

however, in orsm'.py the function _parse_leg() (inner function of parse_osrm_json()) constructs Match objects with distance=None. this is both a mypy error and can cause an error in functions like compute_cutting_points()

discussed with @nreinicke and decided to change the None to float('infinity')

[nreinicke]

Included with #90