Reorder imports and delete unused imports

nifty5/dobj.py

nifty5/domain_tuple.py

nifty5/field.py

nifty5/logger.py

nifty5/minimization/conjugate_gradient.py

nifty5/minimization/descent_minimizer.py

nifty5/minimization/energy.py

nifty5/minimization/energy_sum.py

nifty5/minimization/gradient_norm_controller.py

nifty5/minimization/iteration_controller.py

nifty5/minimization/l_bfgs.py

nifty5/minimization/line_energy.py

nifty5/minimization/line_search.py

nifty5/minimization/line_search_strong_wolfe.py

nifty5/minimization/minimizer.py

nifty5/minimization/nonlinear_cg.py

nifty5/minimization/quadratic_energy.py

nifty5/minimization/relaxed_newton.py

nifty5/minimization/scipy_minimizer.py

nifty5/minimization/steepest_descent.py