ENH: Make plot_displacements more flexible; change default arrow appearance.

[danielballan]

This reworks plot_displacements, a barely-documented function that makes plots like this:

The most important feature added is the optional scale argument, which makes it possible to study small displacements by uniformly enlarging the arrows to some reasonable size.

Also, I replace matplotlib's arrow axis method with the more general annotate. Strangely, arrow gives much less control over the look over the arrows.

Note to selves: The whole plots file needs some attention -- PEP8 and general housekeeping. I only lightly edited it when merging it in from my mr repository. After we merge Rebecca's #104 I will do some cleanup.

[tacaswell]

We have a plots.py ?!

Why don't you use quiver, it take care of most of these details for you and is also a colormappable so you can set the colors via a cmap + array.

[danielballan]

Haha. OK, I'll look at quiver and bump the issue.