Fitting constrains parameters to be positive
Return timecourses from bid_bim_nbd fits

- Modified FitResult class to store fitted parameter values in a dict by
  parameter name, and each instance of FitResult also stores the time
  points and the model prediction trajectory.

- Added common-sense initial guesses for parameter values for 2 and
  3-conformation fits.

- When the fits do not converge the covariance matrix returned by
  scipy.optimize.leastsq is singular, and hence None is returned. The
  function _mean_sd for reporting parameter standard errors now deals with
  this by putting NaN in for the standard error value.