add support for relative scaling factor to cost/distance for additive logit models (MNL etc.)

[MarkRaadsen]

currently this is not yet supported but it means that the scale parameters in for example MNL is strange as it gets interpreted differently per OD. It should ideally be based on a normalised value, meaning that for the actual utility it should be adjusted, i.e., store a separate scale parameter per OD to make this possible