implement color by chain
merge residues of interspersed chains into one chain

residues of a particular chain are not required to be listed
next to each other. Residues of chain A, might be followed by
chain B, which are then again followed by residues of chain
A. Make sure, no duplicate chains are inserted when loading
from the PDB file.
fix fast recoloring for selections

there are still a few issues with coloring of interpolated traces,
e.g. sline, tube and cartoon. These issues will be addressed in
a next commit.