Possibility to thin only a percentage of the production

[chiarazampolli]

@ddobrigk , @mfaggin , @catalinristea , @alcaliva
@ddobrigk , @mfaggin , please take a careful look. I will merge upon your confirmation.

If we want to add a metadata at the same time, for the case where we thin, let me know what I should use.

== Update ==
As discussed below, the use of metadata is not ok, there might be the need for deeper changes. So the second commit forbids that we can use it for now. The lines that forbid it should be removed when things are mature.

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[mfaggin]

thanks @chiarazampolli , the logic looks ok to me. Regarding metadata I've no experience, last word to @ddobrigk for it

[chiarazampolli]

As discussed in the mail thread, we cannot use metadata in case AODs get merged (which is not the case at the moment in data reco, but one never knows). We'll need some further development.
This means that for now the percentage option should not be used. I will add a protection, leaving the code there for when things will be mature.

Chiara

[chiarazampolli]

The second commit avoids that we can use the option.