public NativeRankTypeDefinition getRankTypeDefinition(RankType type) {
        if (type == RankType.DEFAULT)
            type = getDefaultRankType();
        return typeDefinitions.get(type);
    }
Returns a rank type definition if given an existing rank type name,
or null if given a rank type which has no native implementation (meaning somebody forgot to add it),
config-model/src/main/java/com/yahoo/schema/derived/NativeRankTypeDefinitionSet.java