add isautonomous

[ChrisRackauckas]

Checks for whether the ODESystem is autonomous

@shashi is there a better way to do this?

[YingboMa]

LGTM

[shashi]

You can use jacobian_sparsity for speed.

[YingboMa]

jacobian_sparsity will think that x(t) is time dependent.

[ChrisRackauckas]

Yeah. But the method of jacobian_sparsity could be used w.r.t. t and that would speed it up.

[ChrisRackauckas]

Opened an issue in #816 . The issue is that sparsity w.r.t. t still has D(x(t)) as non-zero, so we'd then have to zero all of those, and then use the sparsity. So it's not trivial enough to do right now, but it's something we could think about for the future.