Standardise Metric Naming

[DanTanAtAims]

Coral Cover metric naming should be standard across ADRIA.

• absolute_cover: coral cover in absolute units, e.g., m^2
• relative_cover: coral cover relative to habitable area. Ideally in [0, 1]
• total_cover: coral cover relative to total reef area including uninhabitable. Ideally in [0, 1]

Interfaces loading external data sets should make sure the naming is assigned to the correct data and that any unit conversions are applied.

[ConnectedSystems]

Note for everyone: this is associated with #465, and could be part of the refactor of all metrics to leverage YAXArrays/metadata (#524).

[Zapiano]

I think there might be a confusion when using "absolute" and "total". The word "relative" gives a hint that the cover will be a fraction of something, but it doesn't say "relative to what". Since we have cover relative to the habitable area and relative to the total area, what do you think about relative_habitable_cover and relative_total_cover? Too much?

[arlowhite]

I agree with including "habitable" in the name. relative_habitable_cover 👍
maybe "all" instead of "total"? so relative_all_cover
or what about relative_reef_cover?