Dynamic BABE epoch duration for ĞDev

Since commit f8256087, BABE epoch duration can be set in pallet duniter-test-parameters. I guess it was an idea of @elois so that we can perform tests of the best epoch duration before choosing one for Ğ1. I have no clue why he thought about doing this before Ğ1 migration but for me it’s something we can do later.

I just wanted to write this somewhere to move on.