helper function to convert a list of market.items to an aggregated valuation expression.
itemListToExpression(item.list, market.item.types, market.risk,
standalone = NULL)
a list of marketItem S3 objects.
a character value representing a subset of marketItem classes.
a marketRisk S3 object.
an S3 object of class standalone.
a character value representing the aggregated valuation expression.