changed organization of the solution, to better met future development needs
better solution organization
CargoDeferrer stub
almost fix CargoDeferrer.cs
removed CargoDeferrer.cs since it's still too early for it
DeMorgan laws in Relational Algebra (PredicateExtension)
moved SqlExpression (too early)
introduced ActiveVisitContexts and SourceVisitor (both still to test)
tests for ActiveVisitContext
enter SourceDowncastVisitor
stub SelectionVisitor
stub SelectionVisitor 2
more SelectionVisitor
removed unstable code