Activate ghost variables functionality also for perturbation methods

[matthias-da]

This is an issue reported by Andrzej Mlodak and I think it makes sense to implement:

As I have observed, currently adding ghost variables to an object of the sdcMicroObj class has a sense only if data suppression is planned to be used. However, in my opinion, the ghost variables can be applied also for perturbative method. For instance, if PRAM is used the change of a ghost variable can be performed in the same way as for the key variable connected with it. Similarly in noise addition: the value of noise could be the same both for a key variable and “its” ghost variable.