Adding support for combining track/jet inputs in input var plotting
Summary
This MR introduces the following changes
- Enables the possibility to combine two track variables (or two jet variables) with each other and plot it then (for example, adding nPixHits and nSCTHits).
- Removing the need for a variable dict from the input variable plotting.
- Documentation is added.
- Example combination is added for both jets and tracks.
Tagging @vdao