#' varname: When using tokens, the token field will be updated instead of a computerCodes field
#' varname: When using ud, the ud field will be updated instead of a computerCodes field
#' @param x A single-row data frame, or a string containing the variables and/or values that should be updated (a data frame is converted to a JSON object, strings are stored as-is)
#' @param index The name of the Elasticsearch index to update
#' @param varname String indicating the parent variable that should be updated (when it does not exist, it will be created, all varnames are prefixed by computerCodes)