Visualization: Add option for 'dots' to appear on non-significant grid points on Skill maps

As discussed offline with @ptrascas, currently when the skill metric plots include significance, dots/crosses are drawn over the points that are statistically significant.

The Visualization module should include an option to draw the dots over the non-significant grid points, in case the user wants to avoid masking the color of the significant points.

@nperez @cdelgado what do you think should be the default behavior? Maybe it would also be a good idea to add some text to the image to ensure it is not confusing?

FYI @vtorralba

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