Helper module to check for NaN values in produced ntuples

As per our chats at #94 (closed), @amathad, @jzhuo and @pkoppenb.

We can already use this new module to test some ntuples in DaVinciExamples, DaVinciTests, etc. And we can expand with functions to return a list of branche names containing NaNs, for example. Ideas welcome.

DaVinciExamples/tests/qmtest/tupling.qms/test_davinci_configFuntuple.qmt

@@ -27,7 +27,13 @@
   <argument name="error_reference"><text>../refs/empty.ref</text></argument>
   <argument name="validator"><text>
 from DaVinciTests.QMTest.DaVinciExclusions import preprocessor, counter_preprocessor
+
 validateWithReference(preproc = preprocessor, counter_preproc = counter_preprocessor)
+
 countErrorLines({"FATAL":0, "ERROR":0})
+
+# Check there are no NaN values in the ntuple
+from DaVinciTests.QMTest.check_helpers import has_nan
+assert not has_nan("DV_example_configFuntuple_ntp.root", "Tuple_B0Dspi/DecayTree")
   </text></argument>
 </extension>