Allow for the use of elementary.ParticleID in ENDFtk

[whaeck]

Elementary is an NJOY component that allows us to identify nuclides, nuclei, elements, etc. See here for more information: https://github.com/njoy/elementary

A development version has python bindings for this library.

One of the inconveniences with ENDFtk, is that we need to know the MAT number in order to extract a material from a tape (python pseudo code):

tape = ENDFtk.tree.Tape.from_file( 'some_file_for_U235.endf' )
material = tape.material( 9228 ).parse()

Being able to use an elementary ParticleID would be a big advantage:

u235 = elementary.ParticleID( 'U235_e0' )
tape = ENDFtk.tree.Tape.from_file( 'some_file_for_U235.endf' )
material = tape.material( u235 ).parse()

Using the elementary identifiers elsewhere in ENDFtk (e.g. to extract reaction products, fission yields, etc.) would be a large improvement as well.