Add convenience enumerators for ENDF options 

For example:
- to easily identify resonance formalisms and types: ResonanceType.resolved and ResonanceType.unresolved

We can then write code like:
```python
if resonances.LRU == Formalism.ReichMoore :

    print( 'found one' )
```
instead of:
```python
if resonances.LRU == 3 :

    print( 'found one' )
```