Draft: Update fake factor section + fix a bug in systematics propagation to histos

MR Description

This MR updates the Fake Factors retrieval and application to allow for them to be affected by systematics. I.E. in the original implementation I made, the FFs could not be affected by any systematics, because they were forced in the weights list inside the PrePlot step. The new implementation treats them as any other weight set in the main configuration file, and therefore allows the FFs to be affected by systematic.

To make the implementation of FFs systematics in the standard correction json format, the additional "internal" parameter kwargs["nuisance_name"] is added to the modules run in the PrePlot stage. This allows to make the json files more readable as the systematics will have the same nae used in the main configuration file and in the datcards.

Moreover, a bug is fixed in the define_histograms function where the systematic tag and the direction could be erroneously built. The fix is as simple as moving from split to rsplit on the string to be split.

MR Validation

This MR has been extensively validated as I have been producing many realists with this configuration for around 2 months now. I am setting ti to draft only to gave the time to also open all other related MRs in the CCLUB framework (this should make the review of this MR a little easier).