Introduced DeterministicDesignVariable which projects a distribution to a constant value. Used DeterministicDesignVariable for ThicknessCoverageLayer and SaturatedVolumicWeightCoverageLayer in the scenario where there is no thickness coverage layer. Extracted DesignVariable base class from the (renamed from DesignVariable) PercentileBasedDesignVariable. Related to Issue [WTI-1004] git-svn-id: https://repos.deltares.nl/repos/WettelijkToetsInstrumentarium/trunk@7124 5a3db67b-de53-47b5-99c8-a1c30a6650e2