option to control interpolation behavior for solution objects when using `saveat`

[isaacsas]

It would be nice when one is using saveat to have the ability to tell solution objects an interpolation option to use. For example, currently when using saveat in JumpProcesses and not saving every jump, objects will still use piecewise constant interpolation whereas I would think piecewise linear is more sensible.

Adding a kwarg to call sol(t; interp_method = ...) or such would be a nice way to allow other interpolations while still allowing one to use the solution interface. I could imagine just passing a symbol that identifies the interpolation to use from ones that are currently supported, but perhaps there could be a function-based interface for more generality?