ENDF API fix

Hey folks,

Anthony encountered a bug where the following code

    from pyne.endf import Library
    from pyne import nucname
    u235 = Library("U235.txt")
    xs_data = u235.get_xs(nucname.id("U235"), 16)[0]

Would throw a KeyError. Turns out the data wasn't actually getting read in because I missed a case in a conditional. That's fixed now. 

John