Add particles tally

Updated and refactored the Tally class to address some shortcomings, and added more tests.
@crbates @scopatz you think the rx2fluka etc, should be renamed, fluka, mcnp, mcnp6 respectively and shifted to rxname, which is ok :), but should I instantiate each map the same way that @crbates does with _names for example? Also, should it be _fluka etc? Thanks!