datatypes: split base classes for internal and SECoP datatypes

Simple datatypes used only in properties like ValueType of NoneOr
do not need a couple of methods. Splitting the base class avoids
warnings about unimlemented abstract methods.

Change-Id: Ie7d5754c44a5fb5c3ed8569df544495450347082

test/test_datatypes.py

@@ -75,7 +75,7 @@ def out_of_range(dt, *args):
 
 def test_DataType():
     dt = DataType()
-    with pytest.raises(ProgrammingError):
+    with pytest.raises(NotImplementedError):
         dt.export_datatype()
     with pytest.raises(NotImplementedError):
         dt('')