Support split archives

[yarikoptic]

I thought we had issue already somewhere but failed to locate, so creating a new one.

Use case: crcns/hc-2 dataset where .tar.gz files got split into multiple with suffixes like -a, -b so we fail to add their content

[INFO] Adding content of the archive ./ec014.277/ec014.277.dat.tar.gz-a into annex <AnnexRepo path=/mnt/btrfs/datasets/datalad/crawl/crcns/hc-2 (<class 'datalad.support.annexrepo.AnnexRepo'>)>

command fails (I guess -- surprisingly no other log line)

(git)smaug:/mnt/btrfs/datasets/datalad/crawl/crcns/hc-2[incoming]ec014.277
$> ls -Ll ec014.277.dat.tar.gz-*
-r--r--r-- 1 yoh datalad 3221225472 Oct 28  2014 ec014.277.dat.tar.gz-a
-r--r--r-- 1 yoh datalad 3221225472 Oct 28  2014 ec014.277.dat.tar.gz-b
-r--r--r-- 1 yoh datalad 3221225472 Oct 28  2014 ec014.277.dat.tar.gz-c
-r--r--r-- 1 yoh datalad 1235048799 Oct 28  2014 ec014.277.dat.tar.gz-d

I believe we probably would need to react/process (concatenate, pipe all?) -a file, while looking for any other subsequent one and then ignoring all the other suffixes