Remove unneeded calls to keys()

This is a minor efficiency thing;  When doing invocations of the form `for key in dictionary` or `z in dictionary` or `set(dictionary)`, the `in` uses a built-in iterator of the keys for the dictionary.  If you instead use `dictionary.keys(), you create a list and iterate through that.  That list creation is unnecessary overhead.

An argument could be made, maybe, for added readability from invoking keys(), but that's probably better done by renaming to make dictionaries.  Even then, `set(dictionary)` might be quite confusing to anyone new to python (I only learned you could do set(dict) while doing this...).

All tests passed before I rebased with the current staging branch (after which I removed build/ and re-installed pyne -- is there more I need to do?); The tests that are failing look to all be related to the on-going Surface Source work, and the recent ensdf.py update.