public static Optional<Severity> getMaxSeverity(final Severity severity1, final Severity severity2) {
        if (severity1 == null) {
            return Optional.ofNullable(severity2);
        } else if (severity2 == null) {
            return Optional.of(severity1);
        } else if (severity1.greaterThanOrEqual(severity2)) {
            return Optional.of(severity1);
        } else {
            return Optional.of(severity2);
        }
    }
Get the highest (most severe) severity in the list of severities. If there are no severities,
return {@code defaultValue}.
stroom-util-shared/src/main/java/stroom/util/shared/Severity.java