WIP: Animate polygon -- create animations using a polygon plotting tool

[mrhardman]

This pull request attempts to address the issue highlighted #311 by introducing the ability to make animations with a plotting function based on polygons, not pcolormesh().

This feature is not fully developed and polished compared to what is available through animatplot-ng, but unfortunately that library does not support polygon plotting.

To use this feature on Hermes-3 data, use the following script

#!/usr/bin/env python3

# Following two lines needed so all variables are shown when printing the Dataset
import xarray as xr
import xhermes

xr.set_options(display_max_rows=1000)

# Set better figure size
from matplotlib import pyplot as plt

plt.rcParams["figure.figsize"] = (16,8)

ds = xhermes.open(".", geometry="toroidal",
    gridfilepath="bout.grd.nc",
    keep_yboundaries=False,
    keep_xboundaries=False)

# Get rid of size-1 toroidal direction
ds = ds.squeeze()

# Make an animation
# Note: saving a gif can be slow. Comment out `save_as` argument to disable.
ds.bout.animate_polygon_list(
    ["Nd","Pd"],
    #["Nd"],
    poloidal_plot=True,
    show=False,
    save_as="hermes_animation_new",
)

Please make suggestions to guide the development of this PR. Currently the code is adequate for my needs.

An outstanding issue is the plotting of the separatrix, where the lines do not meet correctly at the X-point, e.g., see below.