Updated constraints on Norm (>0) and Contribution(>0 and <=100). Added TestHelper.AssertExceptionCustomMessage which can be used to assert for custom exception messages. Exposed FailureMechanismContributionItem.Norm. Used more optimal Invalidate() (instead of Refresh()). Updated doc and tests. Related to Issue [WTI-168] git-svn-id: https://repos.deltares.nl/repos/WettelijkToetsInstrumentarium/trunk@1298 5a3db67b-de53-47b5-99c8-a1c30a6650e2