Unused argument `extra_data` in `PlotCollection.map`

[imperorrp]

Very minor issue, just something that caught my eye while looking through this function- the extra_data argument doesn't seem to do anything. It's defined to None by default and defaults to {} if None later. Not mentioned in the docstring either. Was there some intended incomplete functionality behind it?

[OriolAbril]

You can open a small PR to get rid of it. It was an idea for passing arbitrary data via .map but eventually we decided on processing **kwargs directly and subset xarray objects if any of the present kwargs matches the type.