Adjust THREADS_PER_COMMAND for underpacked jobs

[gdevenyi]

Problem arises like this QBATCH_CHUNKSIZE=40 QBATCH_PPJ=80 QBATCH_CORES=40

This results in THREADS_PER_COMMAND=2 regardless of the actual length of joblist.

Instead, this should be adjusted for the length of the joblist if < chunksize