merging BoolenExpressions and some improvements

src/root/Main.java

src/root/ast/expressions/AddExpression.java

src/root/ast/expressions/AndExpression.java

src/root/ast/expressions/BinaryExpression.java

src/root/ast/expressions/BooleanExpression.java

src/root/ast/expressions/DivExpression.java

src/root/ast/expressions/EqualsExpression.java

src/root/ast/expressions/MulExpression.java

src/root/ast/expressions/NegExpression.java

src/root/ast/expressions/NotExpression.java

src/root/ast/expressions/Operator.java

src/root/ast/expressions/OrExpression.java

src/root/ast/expressions/SubExpression.java

src/root/ast/expressions/UnaryExpression.java

src/root/ast/expressions/XorExpression.java

src/root/visitors/EvalVisitor.java

src/root/visitors/GraphvizVisitor.java

src/root/visitors/StringVisitor.java

src/root/visitors/Visitor.java

test/root/ast/expressions/BasicTests.java