Threading control

[choishingwan]

Currently, we can supply a variable to control for the number of CPU required. Is it possible such that when we update the number of CPU required, the task do not rerun? For example, if we have the following

task plink_freq{
    input{
         File bed
         File bim
         File fam
         Int threads = 1
   }
    command <<<
        plink --bed ~{bed} --bim ~{bim} --fam ~{fam} --freq --threads ~{threads}
    >>>
   runtime{
     cpu: "~{threads}"
   }
}

When we update the thread number, the task will rerun because it sees one variable (threads) to be different from previous input. Can we use something like ~{runtime.cpu}? I am guessing this is similar to what is proposed in #418 ?