ENH: add pivot kwarg to 3d quiver plot

Also removing an unused inner function and cleaning up some math.

The default value for `pivot` is "tip", which preserves the behavior with the existing 3d quiver plot, but differs from the regular 2d version which defaults to "tail".

I suspect this PR will need tests added, but I'll need help setting those up.
That is because mplot3d is not pep8 checked. The toolkits generally are not
checked.

On Wed, Oct 29, 2014 at 1:42 PM, Thomas A Caswell <notifications@github.com>
wrote:

> In lib/mpl_toolkits/mplot3d/axes3d.py:
>
> >
> >          linec = art3d.Line3DCollection(lines, *args[argi:], **kwargs)
> >          self.add_collection(linec)
> >
> > -        self.auto_scale_xyz(xs, ys, zs, had_data)
> > +        self.auto_scale_xyz(XYZ[:,0], XYZ[:,1], XYZ[:,2], had_data)
>
> Curious travis didn't catch that...
>
> —
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> <https://github.com/matplotlib/matplotlib/pull/3735/files#r19556818>.
>