Support for histogram filling in oversampled dataset

[arizzi]

Feature description

As discussed in CHEP24 for CMS FlashSim samples produced with
oversampling a dedicated histo filling scheme is needed where events
are first accumulated on same-gen subevents then used to fill a TH
with proper error handling.
In order to achieve this in a (memory) efficient way the following
features would be needed:

• know the number of threads actively used and/or which threads will
  not be used at all or any longer in the processing
• define a global_rdfentry_ that is a progressive number on the inputdataset
• know what global_rdfentry_ a given thread is processing

An additional useful feature, for handling oversampled events is the
option to have output events in Snapshots sorted according to the
global_rdfentry_ (i.e. maintaining the input order)

Alternatives considered

No response

Additional context

No response

[vepadulano]

@arizzi Thank you for the feature request! I am just linking to the related gh issue regarding the global entry number in MT runs #12190