Allow CLI `evolve` to take a generic number of EKOs

[Radonirinaunimi]

Currently,pineappl evolve still only takes up to two EKOs in which the second one is passed through the flag --ekob. This should be generalized to take a generic number of EKOs.

cc NNPDF/pineko#215

[cschwan]

Yes, that was an oversight, thanks for catching it! Due to limitations of clap we should provide a comma-separated list, which looks a bit odd for file names, but I believe that's our only option.

[Radonirinaunimi]

Due to limitations of clap we should provide a comma-separated list, which looks a bit odd for file names, but I believe that's our only option.

Yes, this was also my thinking. I can take care of implementing it.