show sld profile plot ought to go down to R=0

[richardh]

"show sld profile" button ough to produce a plot that starts at R=0 rather than r_core for the core_multi_shell, onion and spherical_sld models.

If sld1 is the sld at r_core, then there ought to be an additional line on the plot from (R=0, sld=sld1) to (R=r_core, sld=sld1)