There was an issue with the unit testing, caused by a recent update t…

…o parcels. The testing data uses only 5 levels, down to 4.7m depth. Particles would cause an out of bounds error in such a case, and be deleted. Updating the density of the particles lets them still sink, but not as quickly as before.

tests/test_kernels.py

@@ -19,7 +19,7 @@ def make_standard_plastictype_settings():
     # Use tiny wind percentage because test data set is not large and wind speeds are quick!
     plastictype_settings = {'wind_coefficient': 0.0001,     # Percentage of wind to apply to particles
                             'plastic_diameter': 0.001,  # Plastic particle diameter (m)
-                            'plastic_density': 1030.,   # Plastic particle density (kg/m^3)
+                            'plastic_density': 1027.67,   # Plastic particle density (kg/m^3)
                             }
     return plastictype_settings