Add more testcases for compute_distance2boundary.

- The method has several paths controlled by conditional
  statements, so for complete coverage of the method, it
  has been parametrized with pytest - four separate test
  cases run through for different paths within.

- This method has been covered. Each and every path that
  leads to completion (return) has been executed.

tardis/montecarlo/tests/test_cmontecarlo.py

@@ -122,11 +122,15 @@ def __init__(self, **kwargs):
 
 
 @pytest.mark.parametrize(
-    ['mu', 'r', 'r_inner', 'r_outer', 'expected'],
-    [(0.3, 7.5e14, [6.912e14, 8.64e14], [8.64e14, 1.0368e15], 259376919351035.88)]
+    ['mu', 'r', 'recently_crossed_boundary', 'r_inner', 'r_outer', 'expected'],
+    [(0.3, 7.5e14, 1, [6.912e14, 8.64e14], [8.64e14, 1.0368e15], 259376919351035.88),
+     (0.3, 7.5e14, 0, [6.912e14, 8.64e14], [8.64e14, 1.0368e15], 259376919351035.88),
+     (-0.3, 7.5e13, 0, [6.912e14, 8.64e14], [8.64e14, 1.0368e15], -838532664885601.1),
+     (-0.3, 7.5e14, 0, [6.912e14, 8.64e14], [8.64e14, 1.0368e15], -259376919351035.88)]
 )
-def test_compute_distance2boundary(mu, r, r_inner, r_outer, expected):
-    packet = RPacket(mu=mu, r=r, current_shell_id=0, recently_crossed_boundary=1, next_shell_id=1)
+def test_compute_distance2boundary(mu, r, recently_crossed_boundary, r_inner, r_outer, expected):
+    packet = RPacket(mu=mu, r=r, current_shell_id=0, next_shell_id=1,
+                     recently_crossed_boundary=recently_crossed_boundary)
     model = StorageModel(r_inner=npclib.as_ctypes(np.array(r_inner)),
                          r_outer=npclib.as_ctypes(np.array(r_outer)))