Fix compile-time name lookup for 2D case.

Author: BeyondEspresso

include/picongpu/fields/background/templates/twtstight/getFieldPositions_SI.tpp

@@ -57,7 +57,7 @@ namespace picongpu
                     floatD_X laserOrigin = precisionCast<float_X>(halfSimSize);
                     laserOrigin.y() = float_X(focus_y_SI / cellDimensions.y());
                     if constexpr(simDim == DIM3)
-                        laserOrigin.z() += float_X(focus_z_offset_SI / cellDimensions.z());
+                        laserOrigin[simDim - 1] += float_X(focus_z_offset_SI / cellDimensions[simDim - 1]);
 
                     /* For staggered fields (e.g. Yee-grid), obtain the fractional cell index components and add
                      * that to the total cell indices. The physical field coordinate origin is transversally