Element finder

Element finder is used to find all possible element lines close to a given energy value.
We can also write it as a function. But I feel that the function find_line is the engine to do the search. So what we need is an interface to wrap it. We can use one function to wrap another. But I just prefer to use class as interface, because it is much more flexible and scalable. It is just my personal coding philosophy, probably not that general.

But some changes need to be made based on your correction.