Implement rounding intermediate values using the new RoundingHelper class.

* Add the $Hooks$Rounding parameter to hold the RoundingHelper
  object (in the constructor, a named list of rounding specs
  can be given, which will be coverted to a RoundingHelper by
  the InsuranceTariff constructor).
* Use the helper in most places where rounding intermediate values
  makes sense
* Each use of the rounding helper is indicated by a separate ID, so
  each rounding occurrence can (and must) be configured separately
* By default, no rounding occurs, unless a rounding accuracy (nr of
  digits) was explicitly configured for the ID
* Add documentation