Issue with HEALPix example

Currently the HEALPix example in

https://reproject.readthedocs.io/en/stable/healpix.html

is problematic because in fact the healpix array is 2D. This causes the reprojection to be very slow as a large (1000,800,1024) array is created and the plotting fails.

We should figure out how to deal with HEALPix files that have vector columns - @lpsinger @cdeil - is that quite a common thing? Currently we have a field= argument for ``reproject_from_healpix``, and I'm wondering whether this should slice the vector column if there is only one vector column?

We should also fix the plot on that docs page to be auto-generated so that we can pick up issues like this (a user complained that the examples on that page didn't work)

Start with the HEALPix example at https://reproject.readthedocs.io/en/stable/healpix.html and the reproject_from_healpix entry point. Reproduce the vector-column case to understand the 2D array and plotting failure, then inspect how the field= argument is handled. Done means vector columns are dealt with appropriately and the documentation plot is auto-generated and works.

Written by the indexing model from the issue text.