Generalized three structures Calculations to StructuresCalculation class. Related to Issue [WTI-594]git-svn-id: https://repos.deltares.nl/repos/WettelijkToetsInstrumentarium/trunk@6151 5a3db67b-de53-47b5-99c8-a1c30a6650e2Former-commit-id: a8405ee298bf3c641f426352c072744f2e3685cbFormer-commit-id: 5968cf3a46e1b19065199c498dbd525ef5f26552