ENH: add support for varying colors in quiver annotations

[neutrinoceros]

PR Summary

Improve wrapping around matplotlib.axes.Axes.quiver's complicated signature to enable the optional 5th positional argument C.

I'm also refactoring QuiverCallback and CuttingQuiverCallback to avoid duplicating code as much as possible.

A 'bonus' change here is that I'm allowing arbitary keyword arguments to be passed down to the wrapped method, because it's much more intuitive to pass cmap="inferno" than plot_args=dict(cmap="inferno"). If requested I can remove this from the PR and possibly repurpose it as a new PR, making similar changes to all callbacks that allow it (i.e. the ones that have only one such dictionary argument)

Another bonus is that I fixed a small bug with the normalize bool flag where users would get a divide-by-zero warning when using normalisation for a field that has 0-len elements.

usage

import yt

ds = yt.load_sample("IsolatedGalaxy")
p = yt.ProjectionPlot(ds, "z", "density")

p.annotate_quiver(
    "velocity_x",
    "velocity_y",
    "vorticity_z",
    normalize=True,
    cmap="inferno_r",
)
p.zoom(40)
p.save('/tmp/')

note that this works with matplotlib 2.2 (oldest supported) as well as 3.5 (newest).
see their old doc here https://matplotlib.org/2.2.2/api/_as_gen/matplotlib.axes.Axes.quiver.html?highlight=quiver#matplotlib.axes.Axes.quiver

for completness, introducing this new "field_c" parameter is technically a breaking change for any user code relying on "factor" being usable as a third positional argument. I personally think it's okay, as long as it's explicitly mentioned in the release notes, because it's impossible to mutate the signature continuously without a breaking change. I'm making all arguments keyword-only to remedy this source of problems in the future.

[neutrinoceros]

Got CI green, but more work needed to allow usage of arbitrary color norms.
edit: never mind this already works using matplotlib.axes.Axes.quiver's norm keyword argument.

[neutrinoceros]

I also would like expose a simple API to associate a smaller colorbar, essentially looking like this:

but doing this correctly necessitates (and gives me a pretext for) a deeper refactor of how colorbars and axis labels work (essentially we need to centralise some bits of code to make them reusable).
This is definitely possible and I don't think that I'm closing any door now if I say it's out of scope for the present PR.