Added support for EMESimulation in ModeSolver plugin

[caseyflex]

Previously, we could add a ModeSolverMonitor to an EMESimulation, or we could convert the scene to a Simulation to use the ModeSolver plugin. Now, we can directly pass an EMESimulation to the ModeSolver plugin.

The change was suggested by @tomflexcompute here: https://github.com/flexcompute/tidy3d/issues/1633

It will also facilitate the planned ModeSimulation classes.