add "--overlap" option to interact script

[matthewabrown]

From "man srun"

   --overlap
          Allow steps to overlap each other on the CPUs.  By default steps do not share CPUs with other parallel steps.

Occasionally, the resources allocated to the shell/terminal on the compute node will interfere when people try to interactively test parallelized workloads (eg. mpirun) because some resources are dedicated to the interactive shell. Adding this option as a default setting probably makes sense.