WIP: Jdeabreu/cocktail shaker

Add new scripts to propagate branching fractions up a chain of decays in a cocktail decfile. This is needed because the total branching fraction for each decay is renormalised to 1 in EvtGen. When the cocktail contains many decay modes and deep chains, this process can be very tedious. This scripts automates the process, starting from the original branching fractions from DECAY.DEC. Examples of pre-propagated decfiles are included.