Allow # of params for init values to be passed in as args

to both nbd_parallel_model and nbd_model_shared. This makes it easy
to experiment with fitting either a single residue's parameters
(e.g., c3 only) versus multiple residue parameters (e.g.,
c3 and c120 simultaneously).
Version of nbd_mcmc.py that fits c3 and c120 only
Increased lower bound for insertion rates to 1e-5
Code to pool steps from multiple chains