Finish the basic structure of Wick multiplication

The basic structure of the Wick multiplication is finished. And the
treatment of be dummy variables from contraction has been redesigned
completely. Now the contraction dummies from the Wick theorem is no
longer going to be explicitly summed / integrated in the amplitude
expression, but rather stored in a special class named by AmpExpr. In
this way, the management of the dummy variables for the summation can be
centralized in the same way as the creation and annihilation dummies.