Add "metric", "recipe" and "assessment area" to the glossary

[ehogan]

Add definitions of "metric", "recipe" and "assessment area" to the glossary in the documentation:

• "metric": a number that quantifies model performance
• "recipe": a grouping of metrics. If metrics are needed for a single plot, they must be grouped together in a single recipe. A recipe may contain one or more keithograms.
• "assessment area": a thematic aspect of model performance; a collection of metrics and plots relating to that thematic aspect, owned by a single domain expert. An "assessment area" contains at least one recipe and multiple metrics.

Context to justify the definitions above, not for the glossary... For the visualisation:

• Each recipe is displayed on its own page. Therefore, if an assessment area contains more than one recipe, each recipe will be displayed on a separate page.
• Each recipe must have a primary plot defined, which is used as a thumbnail. In most cases, this will be a keithogram.

Details

• Estimate: <1 day

[ehogan]

Alistair Sellar (@alistairsellar) would it be possible for you to add the definitions to the first comment in this issue, please? 😊

[alistairsellar]

Definitions now added. Is the level of detail about right?

[Jon-Lillis]

The first half of this issue has clear acceptance criteria which, with no glossary subsitutions needing to be made (unless you have a policy on glossary plurality), I've addressed in 7b465fa. I'm not clear on what is expected for the visualisation section though. Emma Hogan (@ehogan), please could you clarify what was meant here?