"illum_correction.sh": Renaming variables, adding filtering methods and restructured (+ comments)

- LOWERTHROW -> LOWERCUTPERCENT
- UPPERTHROW -> UPPERCUTPERCENT
- Added LOWERCUTRESABS, UPPERCUTRESABS, LOWERCUTMAG, UPPERCUTMAG
- SIGMAWIDTH as individual parameter
- Added loop for combining several cut methods