Update CNOTDihedral class

[ShellyGarion]

Summary

#391

Details and comments

Add the following functions:

• decompose_CNOTDihedral - decomposition of 1&2-qubit elements without look-up tables
• append_circuit - update a CNOTDihedral element inplace by applying a CNOTDihedral circuit
• random_CNOTDihedral - generate a pseudo-random CNOTDihedral element.

Add the following methods to the CNOTDihedral class (so that it will have the same API as the Clifford operator class):

• to_circuit
• from_circuit
• compose
• dot

Update the files:

• dihedral.py
• test_dihedral.py
• init

[ShellyGarion]

With the new functions added, it's now possible to refactor the RB code so that it will not rely on pre-generated group tables save in pkl files (#392).
However, should we first attempt to add the CNOTDihedral class as an operator class in Terra's quantum_info/operators? It seems that we currently have most functionality, and that this class can be useful in other parts of qiskit (in particular to Aqua).

[ShellyGarion]

Closing since this will be finalized as part of #407.