Bugfix for Grad2_par2 on staggered grids

Metric contributions to Grad2_par2 need to be calculated at outloc if outloc!=CELL_CENTRE. This does interpolation each time Grad2_par2 is called which is not optimal: it would be better to calculate interpolated metrics once at all locations (or at least all used locations) and store them somewhere. However, Grad2_par2 does at least return correct results now.