Missing a factor of 2 for spin degenerate system in `postw90.x`

[Chengcheng-Xiao]

In pw90common_get_occ(eig, occ, ef) of postw90_common.F90, a step function is used to calculate the occupation but that step function is maxed at 1. This is perfectly fine for spin polarized or non-collinear calculations but not so for spin degenerate calculations.

For example, the calculated optical conductivity value of a spin degenerate system is halved comparing to the true value.

I could not find any reference to this in both the user manual and the tutorials and just thought maybe it's something that's worth documenting. 🤔