More statistical expression operators

[1ec5]

The iOS and macOS SDKs are representing expressions as instances of the NSExpression class. The style specification currently defines some statistical expression operators: min (minimum), max (maximum), length (count), and + (summation), but NSExpression defines several more predefined statistical functions. To some extent, Objective-C and Swift developers expect to be able to use most of the predefined NSExpression functions without having to consult a large correspondence table.

The following expression operators would improve compatibility with NSExpression:

• avg returns the arithmetic mean (average) value in an array.
• median returns the median value in an array.
• mode returns the most frequent value in an array.
• stddev returns the standard deviation in an array.
• random returns a random number. (Which random number and how this would work exactly is left as an exercise to the implementer. 😉)

mapbox/mapbox-gl-native#10726 synthesizes avg based on +, length, and /, but a native implementation would be preferable.

/cc @anandthakker @tobrun @lucaswoj

[1ec5]

Duplicate of #5853.