'Function to minimize.
Parameters
fps : list
parameters returned by the fitter
args : list
[model, [other_args], [input coordinates]]
other_args may include weights or any other quantities specific for
a statistic
Notes
The list of arguments (args) is set in the `__call__` method.
Fitters may overwrite this method, e.g. when statistic functions
require other arguments.'

def objective_function(self, fps, *args):

    model = args[0]
    meas = args[(-1)]
    _fitter_to_model_params(model, fps)
    res = self._stat_method(meas, model, *args[1:(-1)])
    return res
