First attempt in adding supporting write routines for SurfSrc class
Revert "First attempt in adding supporting write routines for SurfSrc class"

This reverts commit b0b0f4a5217c699fde2dcbac042e744ddd7712f7.
Adding in 2 auxilary methods (write_header write_tracklist) in mcnp.py
and slight mod to check_header test in test_mcnp.py
1st checkin on update_tracklist
2nd checkin on update_tracklist
Improved docstring comments for write_header and write_tracklist
Merge pull request #229 from kmanalo/sswwrite

Awesome work, @kmanalo! Thanks