Add goodsell colour rule

[FilomenoSanchez]

Note: This is waiting for #350 to be resolved.
Note: This is waiting for #354 to be resolved.
It should be possible to create goodsell colour rules. Pastel colors are used for each chain and atoms within each residue are coloured pale / light depending on whether it's the CA or not. The algorithm to get the colours is described in make_colour_table_for_goodsell_style at coot/api/coot-molecules_bopnds.cc.