public void setApproximateThreshold(double threshold) {
        verifyThat(approximateThreshold == null, "already has approximate-threshold");
        this.approximateThreshold = threshold;
    }
This class holds the extracted information after parsing a
rank-profile block in a schema (.sd) file, using simple data
structures as far as possible.  Do not put advanced logic here!

@author arnej27959
config-model/src/main/java/com/yahoo/schema/parser/ParsedRankProfile.java