Fixed integer division bug in generalized_forward_backward
Fixed integer division bug in generalized_forward_backward

There was a division in the calculation of `self.weight` in the `_pre`
function of the `generalized_forward_backward` solver that erroneously
resulted in 0 as a result of integer division when run in Python 2.7.x.
Fixed by changing the numerator `1` to float `1.`

epfl-lts2/pyunlocbox,ThomasA/pyunlocbox,ThomasA/pyunlocbox,epfl-lts2/pyunlocbox