R/calculate_intensity.R: renamed allFeatures variable to featuresGroups

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

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