Capture multiple images for HDR post-processing

[TheNeuralBit]

Capture multiple images with different exposure compensations so we can do HDR post-processing.

From https://islandinthenet.com/hdr-photography-with-raspberry-pi-and-gphoto2/:

gphoto2 --set-config /main/imgsettings/iso=0 
              --set-config /main/settings/capturetarget=1 
              --set-config-value /main/capturesettings/exposurecompensation=0 
              --capture-image 
              --set-config-value /main/capturesettings/exposurecompensation=-2000 
              --capture-image 
              --set-config-value /main/capturesettings/exposurecompensation=-4000 
              --capture-image 
              --set-config-value /main/capturesettings/exposurecompensation=2000 
              --capture-image 
              --set-config-value /main/capturesettings/exposurecompensation=4000 
              --capture-image