Add support for labelling contours

[raphaelquast]

Using m.ax.clabel() to label contour-lines in a plot is currently not working.

After using clabel, the following problems occur

• resulting contours are no longer on the dedicated layer (they are re-drawn and appear on the default ("base") layer
• the initial contours remain in the blit-manager as "artists without a figure"

from eomaps import Maps
import numpy as np
Maps.config(log_level="debug")

x, y = np.meshgrid(np.linspace(-40, 40, 50), np.linspace(-25, 30, 50))
data = x**2 + y**2
data = dict(
            data=pd.DataFrame(
                dict(lon=x.ravel(), lat=y.ravel(), value=data.ravel())
            ).sample(500),
            x="lon",
            y="lat",
            parameter="value",
        )

m = Maps(ax=221, figsize=(10, 6))
m.add_feature.preset.coastline()

m2 = m.new_layer("contour")
m2 .set_data(**data)
m2.set_shape.contour(filled=False)
m2.plot_map()

arts = m2.ax.clabel(m2.coll.contour_set)
for a in arts:
    m2.BM.add_bg_artist(a, layer=m2.layer)

m.show_layer(m.layer, m2.layer)

Describe the solution you'd like
Would be nice to get this working... or to explicitly provide a method to properly label contour-plots.

Issue was reported in the PyOpenSci Review