refactor the way log_increments is set and used

[benoitdes]

Based on our discussion :

• change event argument to private _event argument so that it not defined by user but internally based on data
• make censor argument default value to 0
• define log_increments within GammaProcessData (and remove anything about that from GammaProcess class)
• emove useless if else logic based on increments and log_increments
• remove set and get "log_increments" methods as i dont believe there are usefull anymore
• remove ls_integrate method (we can use the implementation of the parent class)

[benoitdes]

We could also discuss about the possibility to split method _parse_data from GammaProcessData into two parts:

• one for all the check which can raise error
• one for the data manipulation which are useful for latter part of the code