run_telfile: slow merge feature and heavy memory consumption

When attempting to merge large files, the merge function from run_telfile may crash due to excessive memory consumption. This has been reported on the forums by @nicolas.godet: http://www.opentelemac.org/index.php/kunena/scripts/14653-run-telfile-py-merge-with-huge-file-eats-all-ram-and-crash.

The whole process is also quite slow, as mentioned by @noemie.durand in #867 (closed).