Add .downscale(), .upscale(), and .rescale() methods

[VeckoTheGecko]

• Fixes Change grid level #18 (see Change grid level #18 (comment))
• Tests added
• User visible changes (including notable bug fixes) are documented in changelog.md
• New functions/methods are listed in api.rst

Just done downscale for now. Logging off for tonight. Will work on upscale tomorrow (and incorporate #18 (comment) - thanks for the info about hierarchical tooling @keewis , was just looking for that)

Will add tests tmrw as well

[VeckoTheGecko]

Hmm, not sure how expensive this line is with high offsets?

[keewis]

there should be a constant cost (a binary right-shift), so I wouldn't worry too much about it. This implementation doesn't do that, so I'd recommend switching to healpix_geo.nested.zoom_to (nothing for ring, as hierarchy operations don't really make sense there)

[keewis]

I forgot to mention this before, but usually we create new methods on the grid_info objects (in particular, the DGGSInfo base class) and then call that from the accessor. If a implementation doesn't support this operation (or if it just was not implemented yet), that's where you'd raise the NotImplementedError. In this case, though, h3ronpy has a change_resolution function that gives us what we need.

I'd also either return the groupby object, as that would increase the flexibility of the method (if / when xarray supports chaining operations like groupby + weighted or coarsen + weighted we'd then get that for free)

[VeckoTheGecko]

I'd also either return the groupby object, as that would increase the flexibility of the method (if / when xarray supports chaining operations like groupby + weighted or coarsen + weighted we'd then get that for free)

Agreed, I think that sounds good now that we aren't considering rescale (#18 (comment)).

I'm still interested in this issue, but I have quite a bit on my plate with work and can't dedicate much to hobbyist coding. Might only get around to this again end June. Feel free to take over if this is blocking - otherwise I can pick this back up down the line.

[VeckoTheGecko]

Might only get around to this again end June

Scratch that - mainly focussing on work, Rust learning, and other learning so i don't think I have time to get around to this.