estimate_chunks could offer a target_use for chunking

[Zeitsperre]

One idea I had during the workshop was offering a kwarg for target_use when performing rechunking operations. The justification being that users may not know what the idea size (and dimensions, I suppose) are best for their specific use-case:

• Are the zarrs going to be stored/accessed locally?
• Are they being streamed over a THREDDS datacatalog?
• Is the priority on regional coverage (e.g. climatologies)?

We have the possibility of offering a set default along with other settings (e.g. local, thredds, etc.), and setting target_mb could ignore target_use settings by default.