Declare FieldShapeSymbol and FieldStrideSymbol as strictly positive

We can assume that FieldShapeSymbol and FieldStrideSymbol are always positive.

TypedSymbol should forward kwargs to sympy.Symbol.

pystencils/data_types.py

@@ -143,6 +143,13 @@ class TypedSymbol(sp.Symbol):
 
         return super().is_positive
 
+    @property
+    def is_nonnegative(self):
+        if self.is_negative is False:
+            return True
+        else:
+            return super().is_nonnegative
+
     @property
     def is_real(self):
         if hasattr(self.dtype, 'numpy_dtype'):