Add plot() accessors for DataArray and Dataset (#1074)

* Add design spec for hypsometric_integral in zonal.py

* Update hypsometric_integral spec with review fixes

Add column/rasterize_kw params, fix accessor namespace to .xrs,
clarify nodata semantics, specify float64 output dtype, add
list-of-pairs zones support, note dask chunk alignment strategy.

* Add implementation plan for hypsometric_integral

* Add plot() accessors with helpful defaults

Enhance DataArray .xrs.plot() with dask compute, default figsize,
and equal aspect ratio. Add Dataset .xrs.plot() that grids all 2D
variables into subplots with GeoTIFF colormap support.

Author: brendancol