updated pyne for new version of xdress
Cell volume fractions to mixed materials mesh. Method and tests.
PEP8
Docstring clarification.
Masking and other cleanup for PR.
Minor docstring fix.
Merge pull request #346 from elliottbiondo/cell_mixing

Geometry cell material mixing
Merge pull request #343 from scopatz/xd

Updated pyne for new version of xdress