contour levels and lines for nutrient plots

[matt-long]

I should have submitted a PR for the changes I pushed today.

I split nutrient data.ipynb into

• _data-nutrient-plots.ipynb: produces zarr archives of map, zonal-mean, and global profile datasets
• nutrient-plots.ipynb: makes map, zonal-mean, and global profile plots

The plots are nearly there, but I would like to refine the contour levels for the surface maps. For the zonal sections, I used the levels from Sarmiento & Gruber (2006) so I think we're good there.

I think we should aim for a number of levels that yields clear dilineations on the colorbar.

I would also like to add contour lines to all color plots. Labels preferably included. Might need a stride > 1 for color levels.