Merge branch 'master' into byebye_zerocenter

nifty/field.py

nifty/minimization/descent_minimizer.py

nifty/minimization/line_searching/line_search.py

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

nifty/minimization/relaxed_newton.py

nifty/minimization/steepest_descent.py

nifty/minimization/vl_bfgs.py

nifty/operators/diagonal_operator/diagonal_operator.py

nifty/operators/linear_operator/linear_operator.py

nifty/operators/projection_operator/projection_operator.py

nifty/operators/smoothing_operator/smoothing_operator.py

nifty/spaces/lm_space/lm_space.py

nifty/spaces/rg_space/rg_space.py

nifty/spaces/space/space.py

test/test_field.py

test/test_minimization/quadratic_potential.py

test/test_minimization/test_conjugate_gradient.py

test/test_minimization/test_descent_minimizers.py

test/test_spaces/test_lm_space.py

test/test_spaces/test_rg_space.py