Changing a grid's scale_factor

**Description of the problem**

Not sure if this is a bug or not but it is certainly not straightforward to do or figure out, and I should have an inside track on this...

I have a netCDF short int grid with values that need to be scaled by 0.01 to be used. OK, so I can plot it as grid.nc+s0.01. But this data will never be used as is, always with that scale, so better to insert the 0.01 scale into the grid. I have tried various schemes with grdedit and grdconvert but no success. Making this issue mostly so it is not forgotten and once a solution (bug fix, improved documentation) happens then we can delete it.

One can experiment with this grid whose z values should be 0-100 on output, but that requires +s0.01 when used.

`gmt grdmath -R0/10/0/10 -I1 X Y MUL 100 MUL = t.nc=ns`