Fix bug when using <nuclides>all</nuclides> with a void material present.
Added new test in test suite to check for <nuclides>all</nuclides> with void
material.
Merge branch 'develop' into fix-all-nuclides-void
Fixed tallying for specific <nuclides> in a void material. The earlier change
only fixed the special case of <nuclides>all</nuclide>.
Added additional check for negative values in URR probability tables
Updated test_tally_nuclides to include a tally containing a <nuclide> filter
with only one nuclide.
Merge branch 'develop' into fix-all-nuclides-void
Added try/finally in test_tally_nuclides.py.
Merge pull request #279 from mit-crpg/negative_urr_vals_check

Added additional check for negative values in URR probability tables
Merge branch 'develop' into fix-all-nuclides-void
Remove above room temperature data from get_nndc_data.py. Closes #281 on github.
Merge pull request #280 from mit-crpg/fix-all-nuclides-void

Fix tallying <nuclides> with void materials
Change command-line help flag to -h.