Add Dataset.plot.streamplot() method

[johnomotani]

Since @dcherian added Quiver plots in #4407, it's fairly simple to extend the functionality to streamplot().

For example (copying from @dcherian's unit test setup)

import xarray as xr
from matplotlib import pyplot as plt

das = [
    xr.DataArray(
        np.random.randn(3, 3),
        dims=["x", "y"],
        coords=[range(k) for k in [3, 3]],
    )
   for _ in [1, 2]
]
ds = xr.Dataset({"u": das[0], "v": das[1]})
ds["mag"] = np.hypot(ds.u, ds.v)
ds.plot.streamplot(x="x",y="y",u="u",v="v", hue="mag")
plt.show()

• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[mathause]

Looks good. Could you fix the conflict then I am happy to merge. Unless @dcherian wants to have a look?

[johnomotani]

Thanks @mathause - I've merged master now.

[dcherian]

Thanks @johnomotani & @mathause