Add convenience enumerators for ENDF options

[whaeck]

For example:

• to easily identify resonance formalisms and types: ResonanceType.resolved and ResonanceType.unresolved

We can then write code like:

if resonances.LRU == Formalism.ReichMoore :

    print( 'found one' )

instead of:

if resonances.LRU == 3 :

    print( 'found one' )