Support FluxMonitor natively in adjoint plugin

[tylerflex]

currently done through FieldMonitor.flux but would be perhaps useful to support the monitor directly, if only to do this under the hood.

[tylerflex]

passing the torch... actually might not be a bad exercise though. Basically what we will want to do is add some kind of "auxiliary monitor" to the forward simulation, handled similar to the grad and grad_eps monitors, but placed at monitor locations that might need field data (ie the FluxMonitors). Then when we construct the adjoint simulation, we swap in the auxiliary monitor's .flux method for the FluxMonitor data and pretend that this is what the user did.