qutip.piqs notebooks consistency of factor 0.5

[nathanshammah]

In the text and tables of the qutip.piqs notebooks there is sometimes a confusing factor 1/2.

Both Dicke.lindbladian() and Dicke.liouvillian() actually implement
$\gamma L[A] =\gamma ( A \rho A^\dagger -A^\dagger A \rho -\rho A^\dagger A)$
,
such that defining a rate ($\gamma$) as in the text gives the corresponding Liouvillian / Lindbladian.