Quantum circuits with real gates

[DiegoGM91]

I think an interesting feature we may consider including in Qibo is that of changing simulation precision not only between complex128 and complex64, but also enabling float64 and/or float32 in some cases.

This can be interesting for circuits where only real gates apply (e.g. Ry rotations and CZ), which happen sometimes in real-world applications, as in the case of the Text Quantum Classifier or the circuits studied by Carlos et al. in Scaling of variational quantum circuit depth for condensed matter systems.

This is related to #257