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.