Scatterplot: Add opacity slider

[danicahelb]

Discussed in 11/16 data viz mtg

currently scatterplot uses open circles, but circle outline colors are difficult to distinguish. As gradient color map gets implemented, it is increasingly difficult for users to differentiate hues

Add an opacity slider to the plot controls.

• Markers should be filled unless the fill is completely transparent, then switch to hollow markers.
• Default should still be hollow markers, but users can use the slider to add the fill color to increase the colored area
• allowing for opaque filled markers will help address overplotting of filled markers

[danicahelb]

duplicates https://github.com/VEuPathDB/web-eda/issues/1487, so i am closing this one