Add potential density contour to zonal velocity through DP in ocean metrics recipe

[thomaswilder]

Short description of the diagnostic
We propose the addition of the sigma2 density contour (referenced to 2000 m) to the zonal velocity transect through Drake Passage (Fig. 355 Figure 4) in the ocean metrics recipe. For an example, see Figure 3 in Storkey et al., (2025).

To Do

• Branch and pull request
  • Once you've started working, add the branch (and pull request)

@valeriupredoi @rswamina

[valeriupredoi]

cheers @thomaswilder 🍺 - good folk to have around for oceans expertise are @TomasTorsvik @SophieHall2024 @axel-lauer @malininae