Contract: Variable quantity alytics Account Formula

This module adds a new Formula (quantity) for variable quantity in contract. This formula is intended to be used when one wants to use a contract, to onward bill a invoice.

This Formula will look in all posted in-invoices for any line that refers to: the same produkt as in contract line, the same analytics account as in the contract line, that has not been invoiced by this formula before, and that is from before the contract lines end date.

To know if a invoice line has been invoiced already, a link between invoice lines and contracts as been created. This field will be set when the formula fetches the quantity from a invoice line. This is to ensure a invoice line's quantity is only fetched once, and thus not billed onwards more then once.