API: make CachedAccessor public in addition to register_dataframe_accessor

[jorisvandenbossche]

In GeoPandas we added a plot accessor similar to the pandas one (so you can still access all the pandas plotting kinds, also from a GeoDataFrame where the default plot is a spatial map), see geopandas/geopandas#1465.

But, in that PR, we made use of pandas.core.accessor.CachedAccessor, which is how we add accessors in pandas under the hood:

pandas/pandas/core/frame.py

Lines 9730 to 9731 in ffa6e20

	# Add plotting methods to DataFrame
	plot = CachedAccessor("plot", pandas.plotting.PlotAccessor)

So in GeoPandas we basically copied the above code, but with our adapted GeoPlotAccessor.

The reason that we can't use the public way to add accessors with pd.api.extensions.register_dataframe_accessor is that we are overriding an existing accessor of pandas (intentionally).

So can we consider CachedAccessor to be "public"-like so it is fine for GeoPandas to use it? Or do we want to explicitly expose the class in the pandas.api.extensions namespace? Or can we change register_dataframe_accessor to allow it to override existing methods? (not sure we want to make that too easy)