Add ``U``, ``V`` and ``C`` setter to ``Quiver``

The ``U``, ``V`` and ``C`` values of the `~matplotlib.quiver.Quiver`

can now be changed after the collection have been created.

.. plot::

:include-source: true

fig, ax = plt.subplots()

X = np.arange(-10, 10, 1)

Y = np.arange(-10, 10, 1)

U, V = np.meshgrid(X, Y)

M = np.hypot(U, V)

qc = mquiver.Quiver(

ax, X, Y, U, V, M

)

qc.set_U(U/5)

ax.add_collection(qc)

ax.autoscale_view()