ENH: New fsl_sub_wait command

Hi @duncan, I thought it would be nice to have a self-contained command which simply blocks while waiting for other jobs. This would make certain operations in scripts nicer, e.g.:

job_id=$(eddy ...)
fsl_sub_wait ${job_id}

The way that the fsl_sub_wait command works is:

1. Creates a temporary file temp
2. Calls fsl_sub --jobhold <jobs> rm temp
3. Spins until temp has been removed

Note that the semaphore/hold file is being stored in the user home directory, as /tmp/ is not necessarily shared across nodes.

What do you think?