'(list of str) List of human readable parameter names (for parameters
actually included in the model).'

@property
def param_names(self):

    if hasattr(self, '_param_names'):
        return self._param_names
    else:
        try:
            names = [('param.%d' % i) for i in range(len(self.start_params))]
        except NotImplementedError:
            names = []
        return names
