Add multicore support for APEL

As discussed on the lcg-tech mailing, it is required to be able to set the value of batch->parallel to true if the cluster offers multicore ressources. It fixes the #116 issue.
By default, it does not change the configuration (the option is set to false).
The default should be *true*. In fact it should be true in the package and we should not have to specify it explicitely, as there is no drawback to set it to true (single core job will be processed properly).