refactor PDB loading

The PDB loading logic has been split into smaller chunks which
makes it easier to extend. No intended functionality change.
connect nucleotides
add .csf files to gitignore
first steps towards RNA/DNA trace rendering

DNA/RNA are now properly recognized as traces and displayed as such.
It's working for all the trace-based rendering styles. Cartoon does
not look the way it's supposed to yet, because the bases are not
shown this will be addressed in the next commit
each geometry can have multiple vertAssocs

This is required for DNA/RNA cartoons which are partly trace-
based and partly atom-based.
remove a console.timeEnd
PDB: stop after first ENDMDL/END
add sticks for nucleotides
update README
speed up recoloring of DNA/RNA cartoon

Recoloring RNA/DNA cartoons of structures with multiple
chains was slower than recalculating the whole geometry.
By (a) only calculating the color on the N1/N3 atoms and
(b) avoiding to calculate color of the same atom more
than once, we are back to reasonable speed.
silence two grunties
increase thickness for DNA/RNA
initialize to null
silence warning in Chrome