ClusterTask should optionally save job stdout/stderr to member variables

[knrafto]

Right now, the job stdout/stderr goes to /dev/null, but this means it's not a drop-in replacement for ProcessTask in this case.

[nicmcd]

I propose we revamp ProcessTask then make ClusterTask match. I do believe ClusterTask should be a drop in replacement for ProcessTask (whether or not inheritance is used). In general, when dealing with stdout and stderr, I think there are 3 main use cases:

1. Stdout and stderr are not tracked or wanted whatsoever. Only execution and return code. In this case it would be unwise to store std{out,err} in the task anyway (memory space bloat).
2. Stdout and stderr are tracked but kept in the task object, not written to files.
3. Stdout and stderr are written to files.

ClusterTask has another output stream (call it run{out,err} for now) that is the output streams of the runner application that schedules the cluster job in the cluster scheduler (e.g., "srun" for Slurm, "qsub" for SGE, etc.)