Adding a crosshair in the output PDF's.

[Toodlepoodle]

Similar to the google earth image if we can have a method of pointing at a particular lat-long via a crosshair whose size we can manipulate as per our needs within the bounding box in the pdf generated by the plot_datacube function then it would be easier to understand where certain projects are located in the nighttime lights pdf that is generated by the plot_datacube function.
The function would need four new arguements . 1) crosshair- latitude 2) crosshair - longitude 3) crosshair - size 4) crosshair -color