Major update to 'fit_betasbp_cut.py'.
Major update to 'fit_betasbp_cut.py'.

fit_betasbp_cut.py:
* replace getopt with 'argparse'
* added 'get_parameter' to process model parameter initial value and bounds
* support read parameter bounds from input file
* added options '--s0', '--rc', '--beta', '--const' to get paramter
  initial values and bounds
* renamed 'fit_beta_model' to 'fit_model', and added argument 'func' to
  support other models

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