Merge pull request #6 from pdijksta/fix_is_None

== to is

Author: giadarol

FiniteDifferences_ShortleyWeller_SquareGrid.py

@@ -260,7 +260,7 @@ def __init__(self,chamb, Dh, sparse_solver = 'scipy_slu', tol_stem = 0.01, tol_d
     #@profile    
 	def solve(self, rho = None, flag_verbose = False):
 
-		if rho == None:
+		if rho is None:
 			rho = self.rho
 		self._solve_core(self, rho)