Fictitious jump methods for time-dependent rates

[isaacsas]

There are thinning approaches that allow SSAs to "exactly" handle time-dependent rates by adding a fictitious jump to the system that makes the overall jump rate constant. Perhaps we could add support for one of these methods?

When I have some time I'll poke around for references, but please feel free to post any you might know of.

[ChrisRackauckas]

@rveltz posted #7 which links to http://rveltz.github.io/PDMP.jl/latest/ that describes these kinds of methods. We just need to have the VariableRateJump build these kinds of things when requested.