Adding histograms to new plotting python API

Summary

This MR introduces the following changes:

• support for histogram plots with the new plotting python API
  • added histogram.py which builds on top of plot_base
  • added example scripts which use dummy data
• fix typo in ROC plot docs

Relates to the following issues

• first step towards #147 (closed)
• #100 (closed)

Examples

The goal is to keep the code as general as possible, such that all the plots we currently produce with umami can be simplified while at the same time providing flexible code for scenarios where people want to deviate from the default plots.

The code for the following example plots is stored in examples/plotting and put in the documentation.

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides

ToDos

• implement first version
• add unit tests
• improve errors messages / logging in histogram plots
• add support for histtype parameter of hist()
• add documentation
• add simple example scripts (maybe in combination with documentation)