Properly implement premium refund through the params$ContractData$premiumRefund variable

Currently, there was the premiumRefund switch (0/1-valued) and an additional premiumRefundLoading for an additional loading. Both could/should be combined into one premiumRefund paramater that gives the proportion of total premiums paid so far (0=no refund, 1=full refund, 1.05=5% additional death risk sum).