Improve flattening by testing and merging an existing fork

[draeger]

The flattening algorithm implemented in JSBML should be improved. A better version has been implemented in the form of a fork. It is nearly ready but requires more extensive testing before a pull request to the master branch can be issued.

[luciansmith]

There are test models in the libsbml source you would be welcome to use; a few at

https://github.com/sbmlteam/libsbml/tree/development/src/sbml/packages/comp/extension/test/test-data

and many more at

https://github.com/sbmlteam/libsbml/tree/development/src/sbml/packages/comp/util/test/test-data