Use plotting python API in input var plots (track variables)

Summary

This MR introduces the following changes

• Using the plotting python API for the track input var plots (jet variable plots will be updated in another MR)
  • reduced amount of code by using the plotting python API
  • made plot_input_vars_trks also cover the case of multiple input datasets --> removed plot_input_vars_trks_comparison

Relates to the following issues

• First step towards #147 (closed)

Notes:

• The control plots for jet variables also show up in the diff, but this is only due to the change to only legend column (new default value). The histograms themselves are untouched for jet variables.

Conformity

• Changelog entry
• [ ] Documentation
• Development guidelines
• Style guides