def update_distribution(param, value)
      mean = @ranges[param].index(value)
      range = @ranges[param]
      sd = self.sd_for_param(param, range)
      @distributions[param] = Biopsy::Distribution.new(mean,
                                                      range,
                                                      @sd_increment_proportion,
                                                      sd)
    end
def update_distribution(param, value)
      mean = @ranges[param].index(value)
      range = @ranges[param]
      sd = self.sd_for_param(param, range)
      @distributions[param] = Biopsy::Distribution.new(mean,
                                                      range,
                                                      @sd_increment_proportion,
                                                      sd)
    end
set the distribution for parameter +:param+ to a new one centered
 around the index of +value+