Support for single multi-node task

[BoPeng]

Related to #476 and #1277

Currently nodes is one of the task options that has not been implemented. This option is supposed to specify the number of nodes a single task runs, and a template is supposed to do something like

%PBS -l {nodes}:{cores}

sos execute task_id

to execute a multi-node task with id task_id.

However, because multi-node jobs require special way to start (e.g. mpiexec or mpirun) and sos execute cannot mimic this behavior, tasks are currently limited to single node only.

With the new -e option

sos execute -e pbs {task}

we are supposed to implement a PBS version of the task executor. Although this feature is designed to distribute multiple subtasks to multiple nodes, it would be nice to support single multi-node as well. This would involve the submission of a MPI job as follows:

task: nodes=5, cores=4
mpiexec ...