Allow (premium|benefit)FrequencyLoading to be a function or a fixed value; set premiumFrequencyOrder to -1 if loading is given

The (premium|benefit)FrequencyLoading parameters are evaluated using a new function evaluateFrequencyLoading, which handles functions, named lists, and fixed values.

Also, set premiumFrequencyOrder automatically to -1 (i.e. no adjustment due to multiple payments per yer) when a loading is given.