Work out the functionality of repr_outl_as_nan of the combine_all_to_obsspace()

[amberJ99]

According to the documentation, the function combine_all_to_obsspace gives you the possibility to change the values of the outliers to NaN (with the parameter repr_outl_as_nan). But this is not yet implemented in the function (this is mentioned as TODO on line 1929 of dataset.py).