Implement sumInsured calculated from given premium

* Add parameter premium, which defines the prescribe premium (either written, written_beforetax or gross)
* The sumInsured will be derived from the premium by reversing the formulas (as far as possible)
* If both sumInsured and premium are given, the premium is ignored and sumInsured takes precedence
* The function calculatePremiums now also returns the sumInsured (in addition to the premiums and the coefficients for the premium calculation)

Implements #49, documents #63