Enforced margin for two-sided Heaviside to be non-negative.

Initial_Condition.py

@@ -767,6 +767,11 @@ class HeavisideTwoSided(InitialCondition):
         config : dict
             Additional parameters for initial condition.
 
+        Raises
+        ------
+        ValueError
+            If margin is negative.
+
         """
         super()._reset(config)
 
@@ -776,6 +781,9 @@ class HeavisideTwoSided(InitialCondition):
         self._right_factor = config.pop('right_factor', 2)
         self._margin = config.pop('margin', None)
 
+        if self._margin is not None and self._margin < 0:
+            raise ValueError('The margin has to be non-negative.')
+
     def is_smooth(self):
         """Returns flag that function is not smooth."""
         return False
@@ -816,9 +824,9 @@ class HeavisideTwoSided(InitialCondition):
             Value of function evaluates at x-value.
 
         """
-        x_shift = -mesh.cell_len/3 if self._margin is None else self._margin
+        x_shift = mesh.cell_len/3 if self._margin is None else self._margin
         return self._left_factor \
             + (self._middle_factor - self._left_factor) * np.heaviside(
-                x - x_shift, 0)\
+                x + x_shift, 0)\
             + (self._right_factor - self._middle_factor) * np.heaviside(
-                x + x_shift, 0)
+                x - x_shift, 0)