refactored param name and added option

. renamed detect_method to correct_method (name could lead to error in
interpretation)
. added new method to correct_method: percentile (same value used in the
comparison/detection)
. small documentation improvement in homog.py
. fixed skewness method, wasn't using user value
. fixed median method, wasn't calculating fixvalues
ui option to percentile correction method, threading work on progress

. added option on correction method combobox
. still trying to implement threads (incomplete)