Implement contract$addExtension to add a contract extension after maturity (both premium-free and with regular premiums)

* Allow premiumPeriod=0, indicating no premiums are paid (so an initial capital must be given)
* Fix aggregating values of child-blocks that go beyond the main contract's duration
* move sumInsured calculation to its own function for cleaner premium calculation. That function will be called earlier, so the CF data frames already use the correct sumInsured
* The additional_capital cash flow is no longer relative to the sumInsured, but gives the absolute capital