In Octave ≤ 5, it was impossible to take a function handle for a function in a package (“+” folder). This bug has been fixed in Octave 6, so we can use the same codepath as MATLAB in that case. See https://savannah.gnu.org/bugs/index.php?46659
Also added tests to check that we are able to identify the Balanced Growth Path, note that it does not work with tests/bgp/fs2000.mod.