GDS export wrappers in `JaxSimulation`

[tylerflex]

Seems better to just allow JaxSimulation to export simulation directly rather than making the user convert to a regular Simulation first.

# Convert the final adjoint simulation to a regular one.
sim_final = sim_adj.to_simulation()[0]

# Export the final simulation to GDS.
sim_final.to_gds_file(fname="inverse_design.gds", z=0, permittivity_threshold=6, frequency=200e14)

becomes

# Export the final simulation to GDS.
sim_adj.to_gds_file(fname="inverse_design.gds", z=0, permittivity_threshold=6, frequency=200e14)