Describe a computed field request
See : https://developers.coveo.com/display/SearchREST/Computed+Field+Parameters
This specifies the field on which the aggregate operation will be performed. This field is typically a numerical value.
This specifies the operation to execute on the field value.
Possible operations: -- sum: Computes the sum of all values. -- average: Computes the average of all values. --minimum: Retrieves the smallest of all values. --maximum: Retrieves the largest of all values.