Dose Factors for Radiotoxicity Calculations

I like either `NaN` or `None` it sorta depends on how you plan to use it. `NaN` can be nice as you can do math with it without failures. If you're going to return `None` it probably makes more sense to raise some sort of data not available error otherwise the error will be raised when the user tries to do math with it (or the user has to check for `None` which isn't the worst thing as long as it's documented). 

tl;dr - handle missing data in the most appropriate way for your intended use case