helper function to convert a list of market.items to an aggregated valuation function.
itemListToFunction(item.list, market.item.types, market.risk,
with.constant = F)a list of marketItem S3 objects.
a character value representing a subset of marketItem classes.
a marketRisk S3 object.
a logical value. Should the expression be with constant or not?
a function representing the aggregated valuation function.