Fix the problem described in issue apolcyn#7

Corresponding error:
AttributeError: 'LineSegment' object has no attribute 'unit_vector'

An incomplete if module causes the problem. It is possible that the value of the variable 'self.length' is 0.0. In that case the attribute ’unit_vector‘ is not defined.

traclus_impl/geometry.py

@@ -95,6 +95,8 @@ def __init__(self, start, end):
             unit_x = (end.x - start.x) / self.length
             unit_y = (end.y - start.y) / self.length
             self.unit_vector = Point(unit_x, unit_y)
+        else:
+            self.unit_vector = Point(10000000000, 10000000000)
 
     def as_dict(self):
         return {'start': self.start.as_dict(), 'end': self.end.as_dict()}
@@ -127,4 +129,4 @@ def __ne__(self, other):
     def __str__(self):
         return "start: " + str(self.start) + ". end: " + str(self.end)
 
-
+