[PWGHF] Add code for cut-variation method

[fgrosa]

This PR includes:

• A new directory for the analysis macros
• A class for the computation of the data-driven (non-)prompt fraction of charm hadrons with the cut-variation method
• A script that uses the class for the cut-variation method
• An example of configuration file
• A module with helper functions to set the plotting style
• A brief README

The code was tested with Run2 inputs for the D+ in pp@13 TeV and it was verified to give the same results (see figures below for one pt interval).

@fcatalan92@stefanopolitano please check the code and see if there is something that has to be changed / improved.
@xinyepeng@lvermunt@strogolo please check whether the structure for the macros is fine with you.
Thanks a lot!

[lvermunt]

Thanks a lot @fgrosa! To me the PR and the structure of the macros look good. I would just specify "(non-)prompt fraction via cut-variation method" in the section header in the README, so (when this README gets long) it's also right away clear for non-experienced people

[fgrosa]

Thanks a lot @fgrosa! To me the PR and the structure of the macros look good. I would just specify "(non-)prompt fraction via cut-variation method" in the section header in the README, so (when this README gets long) it's also right away clear for non-experienced people

Hi @lvermunt thanks for your feedback! I have fixed the title in the README as you suggested 👍

[fgrosa]

@xinyepeng@strogolo@fcatalan92 did you have time to look at this PR? Is it fine for you? Thanks!

[xinyepeng]

@fgrosa Thanks a lot! It's very nice that the cut variation method can be included in O2Physics, I think it's fine as the first version, green light from my side