Quantum state reconstruction driver in python

Created by: gcarleo

This PR removes the C++ driver for quantum state reconstruction and moves it into pure python. As a result of the other optimizations done in v2.1, and also of a more efficient calculation of rotated observables, this version runs about a factor of 2 faster than the old one (at least on my desktop).

@GTorlai please check that this does not break anything (it shouldn't, but just in case).

Finally, to fully address #299 (closed), we are now left only with supervised learning to be moved to python.