Update change bin implementation

Based on many discussions and ideas I've reworked how data is generated in the OM. It is now done dynamically with a function `change_data` inside `ss3sim_base`, but this function can also be called manually or as a case. See discussion at https://github.com/ss3sim/ss3sim/issues/182#issuecomment-65690768.

There is some testing left to do and further refinement of how sampling is done, but for now it is best to merge it back into master.