* Introduced PipingCalculationGroup, to allow the user to define piping calculation groups. ** Introduced IPipingCalculationItem as element type for PipingFailureMechanism.Calculations in order to have that collection contain both PipingCalculation and PipingCalculationFolder instances. *** Implemented extension methods to recursively retrieve all PipingCalculation instances when dealing with IPipingCalculationItem. Related to Issue [WTI-174] git-svn-id: https://repos.deltares.nl/repos/WettelijkToetsInstrumentarium/trunk@986 5a3db67b-de53-47b5-99c8-a1c30a6650e2