Add method to update position of arrow patch

[saranti]

PR summary

Closes #14217. Adds set_data method from FancyArrow that allows the arrow to be repositioned without re-drawing.
Arrow's x, y, dx, dy and width parameters are now stored in private variables so that they can be reused for recreating the transform.

e.g.

import matplotlib as mpl
import matplotlib.pyplot as plt
from matplotlib.patches import Arrow

fig, ax = plt.subplots()
ax.plot([0, 4], [10, 0])

a = mpl.patches.Arrow(2, 0, 0, 10)
ax.add_patch(a)

# code for modifing the arrow
a.set_data(x=.5, y=0, dx=3, dy=8, width=1.2)

plt.show()

See the discussion here #14217 (comment). I don't know what the consensus is on the Arrow class but If this fix is still needed, I'll add a test and a what's new entry.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[timhoffm]

While the arrow patch has fundamental flaws, it's not going to go away soon. So this addition is reasonable.

[story645]

Test please? sorry should read things better :/

[story645]

Suggested change

	a.set_data(x=.5, dx=3, dy=8, width=1.2)
	def update(i):
	a.set_data(x=.5, dx=3, dy=8, width=1.2)

how would you feel about making this a tiny animation to advertise the purpose of the method?

[saranti]

That sounds good. I'll have a go at making an animation

[story645]

Is tedious but I think these need to be documented individually since they come in individually and not as tuples.

[timhoffm]

This is correct as is. numpydoc explicitly allows this:

When two or more input parameters have exactly the same type, shape and description, they can be combined: