Make nuclide/element names case insensitive in input

Right now, nuclide/element names are case sensitive according to whatever is in cross_sections.xml. This pull request makes them case insensitive so that, e.g., C-Nat and c-naT are considered the same thing. Inspired by pyne/pyne#499.

The lower_case and upper_case subroutines have been renamed to to_lower and to_upper and are now functions so that the argument doesn't have to be modified.