Rename the sizes parameter in plot/plot3d to be size

[core-man]

Description of the problem

Both plot and plot3d have the sizes parameter that are used to plot symbols with different sizes. However, size is used in GMT documentation. Shall we rename this parameter to be size? If yes, I think we should do it as early as possible cause this parameter is often used in PyGMT.

Originally posted in https://github.com/GenericMappingTools/pygmt/issues/1076#issuecomment-804986728:

like the sizes parameter (I dont' understand why we add a s at the end) of the plot/plot3d method

plot is one of the oldest wrappers in PyGMT, and the parameters are not carefully chosen. We had some discussions in the GMT Community meeting, and I think we agree that we will change sizes to size.

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See the format of the -S option in GMT plot:

x y [ z ] [ size ] [ symbol-parameters ] [ symbol ]

See the format of the -S option in GMT plot3d:

x y z [ w ] [ size ] [ symbol-parameters ] [ symbol ]

GMT plot	PyGMT plot	GMT plot3d	PyGMT plot3d
z	color	w	color
size	sizes	size	sizes
intensity	intensity	intensity	intensity
symbol	symbol (requested in #1076)	symbol	symbol (requested in #1076)

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Now, sizes is used in the following files:

• src/plot.py
• tests/test_plot.py
• examples/tutorials/plot.py
• examples/gallery/symbols/scatter.py
• examples/gallery/symbols/points_categorical.py
• src/plot3d.py
• tests/test_plot3d.py
• examples/gallery/3d_plots/scatter3d.py

[seisman]

As the sizes parameter is used a lot, we should be careful with backward-compatibility.