Method to extract single distinct, non-null value from a raster (error otherwise)

[nathanjmcdougall]

By analogy with more_itertools.one.

We just want to raise an error if there are multiple distinct raster cell values (or, I suppose, if the raster is empty). If there is exactly one value, return it.

For users, this is mostly useful for migration contexts where a quantity was previously represented as a float but the user wants to migrate to allow it to vary spatially. Then the underlying representation can be changed to a raster but any methods that expect a single float can use .one safely until proper spatial variability is supported in the user's downstream applications.

[nathanjmcdougall]

As pointed out in #422, one isn't really a good name since that is meant to reduce a single value, not a single distinct value.

Also, as the LLM handled in #422, there needs to be a way to handle NaNs. Presumably, ignoring them makes sense (thinking about the context of e.g. a building footprint which has a constant raster value and NaN elsewhere).

So we need a new name