cleanups

nifty/minimization/conjugate_gradient.py

nifty/minimization/descent_minimizer.py

...inimization/iteration_controlling/iteration_controller.py

nifty/minimization/line_searching/line_search.py

...y/minimization/line_searching/line_search_strong_wolfe.py

nifty/minimization/minimizer.py

nifty/minimization/nonlinear_cg.py

nifty/minimization/relaxed_newton.py

nifty/minimization/steepest_descent.py

nifty/minimization/vl_bfgs.py