Offset values are written in all distribution parameter models

In mboostLSS_fit() all offset values of the distribution parameters (mu, sigma, ...) are written in each distribution parameter model.

e.g. The fit object of the mu parameter has a variable "mu" containing the initial offset value, although it is only needed in the other distributions. The value is not changed in the iterations of the algorithm. 
For the other parameters the same problem applies.
