Add started box cox

[a3eille]

We want to have the possibility to transform the observations with a started box cox using (pseudo code):

StartedBoxCox(value, power, scaling):
if(value < 0)
           warning message
if(value <= scaling)
         return value
else
         if(power>0)
                 return scaling * (1 + (((pow(value / scaling, power)) - 1) /power));
         elif (power = 0)
                  return scaling * (1 + log(values/scaling))
         else
                     error

(No transformation between 0 and scaling, then Box-Cox transformation with power parameter applied to the scaled value)

• add started_boxcox function in the code
• GUI :1 new parameters: scaling; if the scaling is negative, assumes that the user uses the box cox, otherwise, assumes that the started box cox should be used.
• harmonize variable names and warnings for the box cox and started box cox