openmc postrun processing updates

I have re-addressed the review comments from !169 (closed) that pertain solely to the processing of the output from openmc. Output is more nicely formatted and added to the .summary file instead of a separate file, the tests now test the numbers directly since the uncertainties package should be available, and tests are run with more batches which should now give consistent results.

I added the missing pymcsim tests to both sets of ci tests so that they are present for when workbench gets updated. We should expect 22 tests to be skipped in the workbench python 2 environment.