overlay_sources: Add sources to dynamic map
In Michael-Stevens-27/silverblaze: Geographic Profiling in R

Add sources to dynamic map

overlay_sources(
  myplot,
  lon,
  lat,
  icon_url = NULL,
  icon_width = 20,
  icon_height = 20,
  icon_anchor_x = 10,
  icon_anchor_y = 10
)

`myplot`	dynamic map produced by `plot_map()` function.
`lon, lat`	longitude and latitude of sources.
`icon_url`	what image to use for the icon.
`icon_width, icon_height`	the width and height of the icon.
`icon_anchor_x, icon_anchor_y`	the coordinates of the "tip" of the icon (relative to its top left corner, i.e. the top left corner means `icon_anchor_x = 0` and `icon_anchor_y = 0`), and the icon will be aligned so that this point is at the marker's geographical location.

source_locations <- mysim$record$true_source
plot1 <- plot_map()
plot1 <- overlay_sources(myplot = plot1,
                         lon = source_locations$longitude,
                         lat = source_locations$latitude)
plot1