build sequential SS chain based on matching CA/CB shifts

didn't bother assigning HNCACB peaks as i/i-1 because it's
implied by the sequential assignments.  oops, this means the
data is now inconsistently assigned, since I had been doing this
for the previous spin systems
make sequence-specific assignments of SS chain, based on S/T/A/G spin systems and locations of those aatypes in sequence