public void dynamic(BigDecimal factor) {
    int parallelism = Math.max(1,
      factor.multiply(BigDecimal.valueOf(Runtime.getRuntime().availableProcessors())).intValue());
    fixed(parallelism);
  }
Computes the desired parallelism based on the number of available
processors/cores multiplied by the {@code factor}.

@param factor to use to determine parallelism
spock-core/src/main/java/spock/config/ParallelConfiguration.java