Added documentation to tomography.py
Added documentation to tomography.py

QISKit/qiskit-sdk-py,ChristopheVuillot/qiskit-sdk-py,QISKit/qiskit-sdk-py,ChristopheVuillot/qiskit-sdk-py,ChristopheVuillot/qiskit-sdk-py,atilag/qiskit-sdk-py,atilag/qiskit-sdk-py,QISKit/qiskit-sdk-py,atilag/qiskit-sdk-py