Check for negative values when calculating cross sections from probability tables

This pull request adds a check when calculating cross sections in the unresolved resonance range from probability tables for negative values of the elastic, capture, and fission cross section and sets them to zero. A user recently [ran into a problem](https://groups.google.com/d/topic/openmc-users/ZzV77lt83G4/discussion) where an ACE file had negative probability tables that resulted in a negative total cross section. The present fix should guarantee that a negative total cross section is never encountered in the URR.