added basic semantic check

M src/root/ast/expressions/BinaryExpression.java M src/root/ast/expressions/BooleanExpression.java M src/root/ast/expressions/ConstExpression.java M src/root/ast/expressions/Expression.java M src/root/ast/expressions/NegExpression.java M src/root/ast/expressions/NotExpression.java M src/root/visitors/SemanticVisitor.java A test/root/ast/expressions/BasicSemanticTests.java