refactor cost per distance in ratios of price

[DanOvando]

Getting cost per unit distance right is tricky when tune_costs = TRUE. The resulting dynamics are a somewhat complicated function of price, biomass, distance, cost to revenue ratio

Seems in particular sensitive to ratio of price parameter to cost per unit distance parameter. Might be good to try and get that in some sort of comparable units as that seems to drive a lot of things.

For example, could you express cost per distance as a multiple of price? So every unit of distance knowkc x% off of the effective price?