Move as_positionfixes.plot method into separate function

[bifbof]

with #490 we will subclass GeoDataFrame and thus this method will override the inherited plot function. Access to GeoPanda's plot tools will then only be possible via some direct calls to the GeoDataFrame object.

GeoDataFrame.plot(pfs)(*args, **kwargs)

This behavior is awkward.
I would suggest removing the override method from the class and putting it in a separate function.
This will also removes the need for the cascading plot functionality currently employed, drastically simplifying the function.
Alternatively, you could rename the overriding methods.