ScipyOdeSimulator ignores use_analytic_jacobian (regression)

[jmuhlich]

I think #395 broke use_analytic_jacobian through a minor typo. In _integrator_process, the block

jac_fn = None
if jac_eqs:
    jac_eqs = _get_rhs(...

Should be assigning to jac_fn inside the block, not jac_eqs. The code as it stands always leaves jac_fn set to None.

I'm in the middle of preparing the final PR for Energy BNG support which requires some changes to this code anyway, so I will try to fix this as I go.

[jmuhlich]

I realize we don't have any tests that verify whether the analytic jacobian is actually used. How could we do that? I thought about checking the number of jacobian evaluations by accessing the internals of the scipy integrator, but that counter is increased even when the internally computed jacobian is used. Even adding a call-counting function wrapper to the jacobian function would be difficult given the _get_rhs / _integrator_process architecture (can't get at jac_fn or even the integrator object). The last resort would be adding extra args to _integrator_process just for this purpose.