VODE/VODE90 has extra copy of Jacobian

[zingale]

In VODE/VODE90, the integrator stores the Jacobian internally as an array, but we also create a burn_t that has a copy of the Jacobian for calling the network stuff. This means that we have 2 copies of the Jacobian sitting around. In BS, we don't, since the BS integrator just uses the bs_t % burn_t % jac version of the Jacobian.

We should eliminate the extra copy either by:

• restructuring VODE's guts to work on the burn_t version
• allowing the burn_t to have a pointer to the Jacobian, which points to VODE's version.

this may be related to #78

[zingale]

this was addressed in #220