Abins Instrument: use NotImplementedError for method stubs

Abins uses a base class for Instruments, which defines a couple of
methods returning None. None is not the right response if these
methods were not adapted to a child class; use NotImplementedError to
mark these more clearly.

scripts/AbinsModules/Instruments/Instrument.py

@@ -20,7 +20,7 @@ class Instrument(object):
         :param  input_data: data from which Q2 should be calculated
         :returns:  numpy array with Q2 data
         """
-        return None
+        raise NotImplementedError()
 
     def convolve_with_resolution_function(self, frequencies=None, bins=None, s_dft=None, scheme='auto'):
         """
@@ -37,8 +37,8 @@ class Instrument(object):
             'auto' should select something sensible.
         :type scheme: str
 
-       """
-        return None
+        """
+        raise NotImplementedError()
 
     def __str__(self):
         return self._name

scripts/test/Abins/AbinsInstrumentTest.py

+# Mantid Repository : https://github.com/mantidproject/mantid
+#
+# Copyright © 2019 ISIS Rutherford Appleton Laboratory UKRI,
+#     NScD Oak Ridge National Laboratory, European Spallation Source
+#     & Institut Laue - Langevin
+# SPDX - License - Identifier: GPL - 3.0 +
+from __future__ import (absolute_import, division, print_function)
+import unittest
+
+from AbinsModules.Instruments.Instrument import Instrument
+
+
+class AbinsInstrumentTest(unittest.TestCase):
+    def test_instrument_notimplemented(self):
+        instrument = Instrument()
+
+        with self.assertRaises(NotImplementedError):
+            instrument.calculate_q_powder()
+
+        with self.assertRaises(NotImplementedError):
+            instrument.convolve_with_resolution_function()
+
+
+if __name__ == '__main__':
+    unittest.main()

scripts/test/Abins/CMakeLists.txt

@@ -9,6 +9,7 @@ set ( TEST_PY_FILES
    AbinsCalculateSPowderTest.py
    AbinsDWSingleCrystalDataTest.py
    AbinsFrequencyPowderGeneratorTest.py
+   AbinsInstrumentTest.py
    AbinsIOmoduleTest.py
    AbinsKpointsDataTest.py
    AbinsLoadCASTEPTest.py