Merge branch 'master' into tests

.gitlab-ci.yml

ci/install_h5py.sh

demos/wiener_filter_hamiltonian.py

nifty/domain_object.py

nifty/energies/energy.py

nifty/field.py

nifty/field_types/field_array.py

nifty/field_types/field_type.py

nifty/minimization/line_searching/line_search.py

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

nifty/minimization/quasi_newton_minimizer.py

nifty/minimization/relaxed_newton.py

nifty/minimization/steepest_descent.py

nifty/minimization/vl_bfgs.py

nifty/nifty_utilities.py

nifty/operators/__init__.py

nifty/operators/composed_operator/composed_operator.py

nifty/operators/diagonal_operator/diagonal_operator.py

nifty/operators/endomorphic_operator/endomorphic_operator.py

nifty/operators/fft_operator/fft_operator.py