Added libcelegans as submodule.

Currently, libcelegans only provides scripts to convert between cosmid,
wormid and gene names.

.gitmodules

 [submodule "qaf_demux"]
 	path = qaf_demux
 	url = git@gitlab.pasteur.fr:bli/qaf_demux.git
+[submodule "libcelegans"]
+	path = libcelegans
+	url = git@gitlab.pasteur.fr:bli/libcelegans.git

libcelegans @ cdbde7d1

+Subproject commit cdbde7d12d9923f05e577ae32fa82c3ab6b2b1f5