Interpolation On: interpolate if the radius attribute exists, or skip interpolation.
Interpolation Off: Check if the radius attribute does not exist with '''lookup_or_add_for_write_span'''. initialize radius with
constant number
Now users could decide to interpolate the initial radius or not. And I moved the code of initialization from ```curves_sculpt_add.cc``` and ```curves_sculpt_density.cc``` into ```add_curves_on_mesh.cc```
