Element finder

Element finder is used to find all possible element lines close to a given energy value.
I'm inclined to use the `Raises` section for when exceptions can come out of _correct_ usage.  Telling people this will raise `ValueError` if you pass in 'cat' for energy is not helpful.   Too much documentation starts to bend back around in usefulness as it stops being read.

To quote the numpydoc guide:

> This section should be used judiciously, i.e., only for errors that are non-obvious or have a large chance of getting raised.