Data structure for element in xrf calculation

Per @tacaswell 's suggestion, the element data is re-organized. The goal is to build a convenient interface to obtain all the fluorescence related information from xraylib. We may want to extend this element interface as an independent package later, because xraylib is not very easy to use, but currently we only focus on internal use.
It may be a bit confusing. Users can choose either element name(str) or z(int). So I just index string or int from the OTHER_VAL array. It actually works, but not neat enough.