OpenMC Cross Sections path option and info
Adds the option for specifying the openmc cross sections library path in the SON input and prints out the xs library information in the summary. With this change I added some precheck logic to make sure the xs path is useable and to default to the environment variable if the input is not set or not valid.
This is the first of several minor MRs to come that involve updating PyMCSim per requests in #88.