Fix NDims during writeDomain

Fixes the bug for writing fields introduced with [Pull #80](https://github.com/ComputationalRadiationPhysics/picongpu/commit/45aaaa88b08f477405c8e50fcb4a087bbf801c96#commitcomment-4286394)

fieldTmp (1 component but in a 3D domain living) should have screwed up the other fields a little due to that mismatch...