Add ssw_combine.py to examples/

This is an update to a script I wrote in 2011 that takes advantage of @kmanalo's recent additions to the SurfSrc class in mcnp.py.

I haven't had a chance to test combined files in MCNP.  I will do this tomorrow evening, probably.

For MCNP6 and MCNPX ssw files, "combining" a single file produces an identical file.  For MCNP5, the resulting file is 4 bytes smaller (if you repeat with the produced file, the size does not change again), but print_header and print_tracklist produce identical strings... I feel like I saw this issue a couple years ago, but cannot recall if I solved it.  I think what the issue is is that the mcnp5_surfsrc.w file in tests/ was generated in Windows (this is a thing I would have done back then), so it has some \r\n characters instead of \n.  The byte differences are in fact in the header, if I compare the two binary files visually.

(disregard the other commits; they're taken care of with my previous pull request)