Releasing backwards tracking PRT example models

[wpbonelli]

We omitted backwards particle tracking examples from the 6.5 distribution, for lack of a way to reverse head and budget files in the packaged example scripts — flopy is not distributed with MF6 or assumed present in the user environment.

There has been some discussion on adding native support for backwards tracking, but we don't have it yet.

To include backwards tracking examples in 6.6 we need a way to reverse output files which does not require python/flopy.