Long range normed concentration for short-range class

The long_range_normed_concentration is the result of the long-range concentration normalized by the viral_load - https://gitlab.cern.ch/cara/cara/-/blob/master/cara/models.py#L1156

This calculation is part of the short-range _normed_concentration method. For consistency, it should be defined in a proper method - long_range_normed_concentration - inside the ShortRangeModel class.