Uses a Gaussian decay around a central location to add a hotspot to the density grid.
add.hotspot(object, centre, sigma, amplitude)# S4 method for Density
add.hotspot(object, centre, sigma, amplitude)
an object of class Density or Simulation
an x,y-coordinate giving the centre of the hotspot
a value giving the scale parameter for a gaussian decay
the height of the hotspot at its centre
the updated Density or Simulation object