Element finder

Element finder is used to find all possible element lines close to a given energy value.
All, thanks for your opinions. Really appreciate it...
I treat this search process as  two steps: 
a) define object with fixed incident energy and element list. those are the attributes of the class, and will not be changed during the searching process. Because xraylib needs those attributes to do calculation to output a full table for us to search. Our search is based on the output. If we do ti as function, we need to call xraylib to calculate every single time. 
b) search process with parameters like energy and diff, those are called parameters different from attributes, because they are frequently changed without affecting the object properties. I may want to add more methods like "findmore" which will do search on multiple lines, and other methods but not fully ready yet.