Fix autoplot: register hvplot xarray accessor and handle single-element plots

[marcosfrenkel]

Summary

• Add import hvplot.xarray to hvplotting.py so the .hvplot accessor is registered on xarray objects (was causing AttributeError: 'DataArray' object has no attribute 'hvplot' on load)
• Fix plot_xr_as_2d to return the plot directly when .cols() is not available (single QuadMesh element has no .cols(), only multi-plot Layout objects do)
• Guard .cols(2) call in plot_panel the same way

Root causes

hvplot requires an explicit submodule import (hvplot.xarray, hvplot.pandas) to register its accessors — import hvplot alone does not do it. Additionally, .cols() is a method on HoloViews Layout objects (produced by +-combining plots), not on individual elements like QuadMesh.