fix rgb.scale/rgb.copy

They were not properly updated when colors were changed from RGB
to RGBA.
parse insertion codes from PDB files

PDB loading has been adapted to honor insertion codes for residues.
It also uses insertion codes for HELIX/SHEET records. There are
still a few other places that do not know about insertion codes.
Prominent examples are the coloring methods or selection by residue
number. Nonetheless, this fixes the worst of #53.