First GitHub-based release – no substantive changes from v1.6

This release only marks the move of SBMLPkgClass to a separate GitHub repository, away from its original location as a subdirectory in the general SBML project area on SourceForge. There are no code or LaTeX differences. Apart from some file cleanup and the addition of a README.md file, the only other difference is that the file LICENSE.txt has been updated. (The changes to LICENSE.txt do not alter licensing or distribution terms.)