extract some common integrator infrastructure

[zingale]

[zingale]

this looks a bit clumsy, so comments welcomed. With this, I can make BackwardEuler and RKC reuse the same integrator_setup

[zingale]

I am not sure how to fix the CUDA issues here. It seems to not like passing the integrator function into the integrator_setup function

[zingale]

update: std::function() doesn't work on the device, but a simple function pointer does.

[zingale]

it appears that function points are not supported by SYCL, so we'll need to use functors (e.g., a struct wrapper with an overload of ())

[zingale]

requires #1541

[zingale]

this has been refactored so it will be SYCL compliant now