added scripts for syncing gitlab>svn and svn>gitlab. the former can be done by anyone by rebasing a branch on the master but be sure to know what to do, see https://gitlab.in2p3.fr/la-communaut-des-mod-les-atmosph-riques-plan-taires/git-trunk/-/issues/13#note_107327. the latter can only be done by a gitlab maintainer.