R/calculate_intensity.R: Some operations in prepareRegions are now multicores

Signed-off-by: Charles Joly Beauparlant <Charles.Joly-Beauparlant@crchul.ulaval.ca>
R/calculate_intensity.R: Added the prepareRegionsPaddings function

Signed-off-by: Charles Joly Beauparlant <Charles.Joly-Beauparlant@crchul.ulaval.ca>