Dose Factors for Radiotoxicity Calculations

The reason I don't like NaNs or Nones is that it forces the downstream user to care about and handle something that in most cases they don't care about.  If I have a material and I want to know the dose rate of that material, the computation should just work.  I shouldn't have to worry about whether or not my material contains nuclides which may or may not be present in the database.  This forces users to worry about data provenance in a way that is pretty frustrating.  As developers, worrying about provenance is our bread and butter but shouldn't be something we foist on others.  Note that there are many other instances in pyne.data such as decay where we assume reasonable defaults for missing data.  I think that is accettable here too as long as it is documented.