Added check on modelPair in Problem.

SimPEG/Problem.py

@@ -1,5 +1,5 @@
 import Utils, Data, numpy as np, scipy.sparse as sp
-
+import Model
 
 class BaseProblem(object):
     """
@@ -39,10 +39,12 @@ class BaseProblem(object):
     counter = None   #: A SimPEG.Utils.Counter object
 
     dataPair = Data.BaseData
+    modelPair = Model.BaseModel
 
     def __init__(self, mesh, model, *args, **kwargs):
         Utils.setKwargs(self, **kwargs)
         self.mesh = mesh
+        assert isinstance(d, self.modelPair), "Model object must be an instance of a %s class."%(self.modelPair.__name__)
         self.model = model
 
     @property