Remove warnings and improve colorbar gallery example (#596)
Remove warnings and improve colorbar gallery example (#596)

* Remove warning about Axis sub-item a/f set more than once
* First round of improving colorbar gallery example
* Wrap colorbar docstring to 79 characters
* Use x+l instead of +L for colorbar label and fix typos

Co-authored-by: Dongdong Tian <3eae6d0b95aef6af8a0d785fe9f127268703c43c@gmail.com>
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python