Simple interface for plot field for both time and frequency

I think the following should work:
- `plot(SimulationResult)` should return several lines plots for each x
- `plot(SimulationResult, linetype=:contour)` should attempt a contour plot. This should work for both `TimeSimulationResult` and `FrequencySimulationResult`. Note the function `run(Simulation,shape)` can return both `TimeSimulationResult` and `FrequencySimulationResult`, which can then be plotted
- `plot(FrequencySimulation, ω)` should generate a contour plot.

There might be a dispatch issue for the first two, as I'm struggling to get plot recipes to do this...