Add ability for a task to capture all its outputs in a read-write ext3 overlay

[prioux]

This might be only practical when the task is a singularity task, but it could be useful for the task's work directory to be configured as a blank r/w ext overlay.

That was we consume a lot less inodes on the cluster side.

As a more refined function, this could also be done, maybe, for the just actual output of the task, if it is well-behaved and the overlay paths are sensible as far as CBRAIN goes.

[prioux]

I made some experiments this weekend, in an existing and completed FMRIPRepSingleSubject task:

1. created two ext3 filesystems:

truncate -s10G work.ext3
truncate -s2G zzz.ext3     # zzz is name of output folder 
mkfs -t ext3 -E root_owner=60003:60003 work.ext3
mkfs -t ext3 -E root_owner=60003:60003 zzz.ext3

2. created symlinks that will work only inside container:

ln -s /cap_work work
ln -s /cap_zzz zzz

3. edited the .science.FMRIPRep_blahblah.sh script to add bindings:

(singularity command)
    -B work.ext3:/cap_work:image-src=/,rw \
    -B zzz.ext3:/cap_zzz:image-src=/,rw \
(rest)