Moved the nightshade implementation into a feature.

[pelson]

Rationale

Creates a Nightshade feature that can be added to a cartopy GeoAxes, rather than a .nightshade method on the GeoAxes. (follow up to #1135)
The reason is one of cleanliness on the GeoAxes API, as well as a better abstraction for future cartopy growth (e.g. alternative backends)

Implications

• cartopy/feature.py moved to cartopy/feature/__init__.py
• cartopy/mpl/_solar.py inlined into cartopy/feature/nightshade.py (solar physics don't need to belong in a matplotlib interface)
• adding a nightshade to a GeoAxes now looks like:

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from cartopy.feature.nightshade import Nightshade


ax = plt.axes(projection=ccrs.PlateCarree())
ax.add_feature(Nightshade())
plt.show()

[dopplershift]

👍 to making this a feature.

[pelson]

OK. Ready for final review I think.

[QuLogic]

One minor fix.

[pelson]

Thanks for the review @QuLogic. Very happy to see this as a feature 👍