Description of the problem

We already have an example for colorbar in the gallery (https://www.pygmt.org/dev/gallery/plot/colorbar.html). However, the example is a little low-quality.

1. The example gives many warnings like:

   colorbar [WARNING]: Axis sub-item f set more than once (typo?)
   colorbar [WARNING]: Axis sub-item a set more than once (typo?)
   

2. The example is not beautiful at all. The main reason is that the figure size is too small and the colorbars are too large.

3. The example is not as usable as I expect. I believe very few people will plot colorbars inside a frame. IMHO, at least we should show

   1. calling colorbar with very few arguments, using the default location and default size
   2. the easiest way (using J) to plot colorbars outside the basemap.

@weiji14 As the example was created by you, I'd like to hear what you think. You can improve the example if you like.