Provide support for descriptions of field value options

[jsoules]

This is particularly noticeable in the helicity field added in the 2024.01 database export, but it'd be good to support descriptive values (for e.g. dropdowns) without having to resort to extra lookup functions or, even worse, indexing into a variable.

Probably the right thing to do is to add it to the Field variable definition and provide something along the lines of the field value description lookup function--right now it's a cheap hack relying on the underlying values being able to index into a descriptor.