add option to plot2D to plot contour of geometry rather than its image

[oskooi]

Fixes #1435.

Adds new options contour and contour_linewidth (for Matplotlib's contour) to eps_parameters of plot2D.

Documentation is also provided.

Example usage:

sim.plot2D(fields=mp.Hz,
           field_parameters={'alpha':1.0},
           eps_parameters={'contour':True},
           plot_sources_flag=True,
           plot_monitors_flag=True,
           plot_boundaries_flag=True)

For comparison, the default imshow rendering:

sim.plot2D(fields=mp.Hz,
           field_parameters={'alpha':0.7},
           plot_sources_flag=True,
	   plot_monitors_flag=True,
           plot_boundaries_flag=True)

[smartalecH]

Looks good on my end.

[oskooi]

Note that the origin keyword parameter of contour is set to lower by default which is different from imshow which is upper. The default origin value for contour must therefore be changed to upper for consistency.