Added preserve_xsid capability

[sbradnam]

I have added capability to retain xs library data from mcnp and serpent input files, which is invoked by a new command line argument, -xs.

[shimwell]

Congrats @sbradnam GitHub it telling me this is your first PR.

Welcome onboard. 🎉

We have some test which are in a non standard location (csg2csg/tests) and really should be moved as they are easy to miss.

However it would be good to add a few tests to check the functionality works as excepted

[makeclean]

Did you get around to adding any tests @sbradnam ?