Automatically set local executor CPU limit

[bfrgoncalves]

Currently, the default number of CPUs used by the local executor is as follows in the nextflow.config file:

executor {
  $local {
      cpus = 4
  }
}

It would be nice to change this behaviour to query for the total number of CPUs available and fill the variable according to it. It could then be replaced in the nextflow.config or set a new one in the resources.config file.

[tiagofilipe12]

But, remember not to use all the cpus available in a local installation but rather a subset of the available cpus (say half?).