Move Bell model unfolding rate test into pysawsim/test/bell_rate.py. Cleaned up model and parameter estimation. See W. Trevor King, Meihong Su, and Guoliang Yang. Monte Carlo simulation of mechanical unfolding of proteins based on a simple two-state model. International Journal of Biological Macromolecules 46 (2010) 159-166. doi: 10.1016/j.ijbiomac.2009.12.001 (king10 in src/sawsim.bib) for details.
Extended bell and const rate tests to multi-domain chains