remove work files after finishing the filter operation
make exclusion list before doing anything else

If the collection as a whole is not publishable then  cloning and
annex-getting the files is a waste of time..
added user-friendly documentation to ddrfilter command help
cleaned up code and comments
rm remotes from final public repo

The final public repo contained a remote pointing to the
temporary filter repository.  This is kind-of pointless once the
temp repo is deleted.