Element finder

Element finder is used to find all possible element lines close to a given energy value.
If we want to do this as a class or a function depends on the use case.  If the expectation is that the users will do many searches with fixed incident energy and fixed search list, then doing it as a class makes more sense.  If the expectation is that users will make one call, throw the object away and then create a new one than a function makes more sense.  

Using a class makes sense when there is some amount of state you want to maintain between calls (or you are matching the interface of an existing class like in the xray lib wrapper, the reason to make a class there is because it makes the look-up functions look like frozen dictionaries which has major usability gains).  

At this point, I think I agree with @ericdill that doing this as a function makes more sense because a) I don't see the state being maintained b) don't fully understand the use-case and c) functions are easier to understand.

One cute thing to do would be to integrate this function back into the Element object and that would be easier to do with this as a function rather than as a class.