Draft: Axis color bar

Description

Add possibility to add a color bar to the axis.

This colorbar can be used for example for the heatmap !347

TODO: currently the axis is bound to the csystem of the CartesianPlot. Find a way to deattach for example by creating a fake cSystem for the Axis, because the axis is actually anymore bound to the CartesianPlot Ranges, but to the ranges in the heatmap.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes:

When external dependencies are removed

• Reporting to ...