Use `jax.grad` to calculate `dmgas_dt` rather than finite difference derivs?

[aphearin]

On this line it looks like dmgas_dt is being computed via finite differences. But @alexalar: since mgas is computed differentiably, shouldn't it be possible to define dmgas_dt using jax.grad instead?

This is currently the only use of the _jax_get_dt_array function in the repo.