Allow passing an array as intensity for plot3d

[core-man]

Description of proposed changes

Currently, we can have two ways to specify an intensity/intensities using the plot3d method.

• Simply give a constant intensity like intensity=0.5, which works for both x/y/z and data input. This is similar to -I0.5 in the GMT command line.
• Set intensity=True, and read intensities from an input file or from a 2D array, which is only for the data parameter. This is similar to -I in the GMT command line.

So, when we want to change the intensities for different points, we can only use the data parameter now. This PR tries to pass an array to intensity so that we can set different intensities when we use x/y/z as point input. This way will not work for data input.

Related to #891

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