Jacobian for system of ODEs

[linas]

Looking forward for your StanCon contribution while Bob’s logistic regression example is quite self explanatory (http://discourse.mc-stan.org/t/map-rect-in-the-stan-language-mpi-is-coming/3547/2). I still suspect that even with map_rect supplying jacobian for ODEs would reduce time even more (simply because calculating jacobian numerically requires multiple solutions of ODEs). 15% from 5 days is significant but I would like to push the boundaries.