colorbar: Add modifier +n|N to -D option for placing colorbar NaN box on the right and to skip label

[PaulWessel]

The new modifier for placing a NaN-color box and NAN label at the end of the color bar is now extended to

+n|N[-|label]

where the new +N means place it on the right side of the bar (top for vertical bars) and if the label is - we do not place any label. Default is still NaN unless you append an alternative label.

Added two new test scripts psscale/colorbar-nans-h.sh and psscale/colorbar-nans-v.sh to ensure it works for both horizontal and vertical bars.

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
added	test/baseline/psscale/colorbar-nans-h.ps
added	test/baseline/psscale/colorbar-nans-v.ps
modified	test/baseline/psscale/

Image diff(s)

Added images

• test/baseline/psscale/colorbar-nans-h.png

• test/baseline/psscale/colorbar-nans-v.png

Modified images

Path	Old	New

Report last updated at commit a1fe9aa

[seisman]

Add the add-changelog label and change the PR title to a more descriptive one like "colorbar: Add modifier +n|N to -D option for placing colorbar NaN box on the right and to skip label"? If so, life will be much easier when we work on the GMT 6.6.0 changelog (xref #8159).