Moved expressions to specific packages

Binary/Unary moved to Expr.Binary/Expr.Unary
Replaced VisitorResult Interface by Generic type
Added numeric expression type
Added numeric answer type
Enhanced output of PrintVisitor
Refactor of expressionVisitor

Group expressions to expression types (Unary, Binary, Type)
Rename of ExprType to Atom

ExprType was not the right name. It represents an Atom!
Rename of AnswerType to Type

AnswerType consisted of pure Types. With the rename of ExprType to Atom
we are able to name it right.
Removed unnecessary postfix "Expr"

Removed from: AbstractBinaryExpr and AbstractUnaryExpr.
Rename of QuestionForm to form.Question

Removed postfix and used namespace to identiy a questionForm.
Rename of IfStatement to statement.If

Removed postfix and used Namespace to identify IfStatement.
Refactor of TestExpressions