specify encoding for python2 compatibility

pre/BCs/inhomogeneous-mass-flux/prepare-inhomogeneous-mass-flux-BC.py

 #!/usr/bin/env pvpython
+# -*- coding: utf-8 -*-
 
 import inspect
 

pre/inhomogeneous-fields/inhomogeneous-permeability-cell/prepare-inhomogeneous-permeability.py

 #!/usr/bin/env pvpython
+# -*- coding: utf-8 -*-
 
 import inspect