Make PyGriffin an optional dependency, provide setup script, and include setup instructions.
This MR makes PyGriffin an optional dependency for PyARC. Unit tests are now skipped if the module isn't present.
I've also added a script to install PyGriffin to the Workbench environment. This is similar to the setup_openmc.sh
and dassh_install.sh
scripts from @kkiesling and @matz.
Finally, I've added a section on installing PyGriffin as part of Workbench or for PyARC standalone execution.