Add average expression

[TomasEng]

Our expression language should have a function that returns the average of a list of numbers.

Acceptance criteria

• We have decided on how null values are weighted in the calculation (should they add to the total number of elements or not?)
• The function exists in both frontend and backend
• The function takes a list of numbers as its first argument and a fallback value for emty lists as its second argument
• The function returns the average of the numbers in the provided list

[TomasEng]

We have agreed that null values should be counted in the total number of elements, so that ["average", list] becomes the same as ["divide", ["sum", list], ["count", list]].