Remove references to x and y in codebase (in favor of lon/lat/depth and i/j/k)

[VeckoTheGecko]

Good idea to add an explicit docstring. Done now in 8ea3c4a

However, as I was implementing this suggestion, I realised that our naming of x and y is actually quite confusing... Perhaps not for this PR, but we might want to do a refactor where we clearly split the code into

• Particle positions - which we call lon, lat and depth (even though they could also be on flat grid, let's call them always the same)
• Grid locations - which I propose we consistently call i, j and k following e.g. Nemo convention. These may or may not be in the x, y and z directions, depending on whether the grid is rotated.

To avoid this confusion, perhaps best to avoid the symbols/variables x and y as much as we can throughout the codebase?

So in this _search_indices_curvilinear_2d() method, we would then rename x and y to lon and lat, and xi and yi to ii and ji (or i_index and j_index).

What do you think, @VeckoTheGecko? Is this a useful refactor? Or is it not important enough?

Originally posted by @erikvansebille in #2211 (comment)