specify direction in eigenmode source in optical forces tutorial

[oskooi]

Fixes #1202.

#1035 changed the default behavior when a 2d cell is combined with a k_point with a non-zero z component from a 3d to a 2d simulation. With this change, the 2d eigenmode source with eig_freq_match=True in Tutorial/Optical Forces was aborting with the error Could not determine normal direction for given grid_volume. To fix this, the direction property of the eigenmode source must be set to z from the default AUTOMATIC. (Note also that NO_DIRECTION will not work.)

The Jupyter notebook and the Scheme version are also updated with this change.

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[stevengj]

In a 2d simulation with a nonzero kz, I think we should be able to automate this.

[stevengj]

Arguable, we should be setting eig_match_freq = False here, because the kz is fixed by the simulation.

If eig_match_freq == False and we have a 2d simulation and a 2d region, with a nonzero kz (β), I think that Meep should automatically infer that the k direction is kz.