Move Bell model unfolding rate test into pysawsim/test/bell_rate.py.

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.