Imported core files from my previous private project

I've started a separate project for public release because my
private project contained copies of part of the TNMR documentation
(specifically pseudo-C-code struct definitions for the .tnt file
format) as inline comments so I didn't have to keep switching back
and forth between code and documentation when implementing the
structs as numpy structured dtypes. To avoid any problems of
releasing TecMag's copyrighted documentation, this new project
has had this material removed before committing to the
repository.