Fixed minimizers and their tests.

nifty/minimization/descent_minimizer.py

nifty/minimization/steepest_descent.py

nifty/minimization/vl_bfgs.py

test/test_minimization/quadratic_potential.py

test/test_minimization/test_conjugate_gradient.py

test/test_minimization/test_descent_minimizers.py

test/test_minimization/test_relaxed_newton.py

test/test_minimization/test_steepest_descent.py

test/test_minimization/test_vl_bfgs.py