added NaN variable

theano/gof/nan_type.py

+from type import Type
+from graph import Variable
+
+class NaNType(Type):
+    def __init__(self, why_nan = '(no explanation given)'):
+        """
+            why_nan: A string explaining why this variable is NaN
+        """
+        self.why_nane = why_nan
+
+    def filter(self, data, strict=False, allow_downcast=None):
+       raise
+    def filter_variable(self, other):
+       raise
+    def may_share_memory(a, b):
+       return False
+    def values_eq(a, b, force_same_dtype=True):
+       raise
+
+
+
+class NaNVariable(Variable):
+
+    def __init__(self, why_nan = '(no explanation given)'):
+        """
+            why_nan: A string explaining why this variable is NaN
+        """
+
+        self.type = NaNType(why_nan)
+