Fix incorrect indexing in IncoherentStokes kernel
For clarity, the write_stokes function in split two: one for NR_INCOHERENT_STOKES=1 and NR_INCOHERENT_STOKES=4
For clarity, the write_stokes function in split two: one for NR_INCOHERENT_STOKES=1 and NR_INCOHERENT_STOKES=4