Add the shading parameter to colorbar() in base_plotting.py

[willschlitzer]

As mentioned in #749, this adds the shading alias/parameter to pygmt.Figure.colorbar.

I'm not familiar with how shading will affect the color bar, other than that it makes it look different and darker, so I left a pretty vague description for it under the parameter list.

[seisman]

Please see https://docs.generic-mapping-tools.org/dev/colorbar.html#i

[willschlitzer]

Didn't think to check the GMT docs on this one; copied the text from the GMT docs and changed the arguments to string or boolean

[seisman]

Hi @willschlitzer more work needs to be done here.

So the possible values of the shading (-I) option are:

• bool: shading=True or shading=False
• a single value. for example shading=0.8 means -0.8 to 0.8
• low/high, for example shading="-0.3/0.8".

For case 3, it would be better if we can also support shading=[-0.3, 0.8], just like we can use region="0/10/0/10" and region=[0, 10, 0, 10].

[seisman]

For case 3, it would be better if we can also support shading=[-0.3, 0.8]

To implement the feature, you need to add I="sequence" to @kwargs_to_strings.

[willschlitzer]

Sounds good, I'll work on this tomorrow