Merge branch 'NIFTy_4' into energy_tests

.gitlab-ci.yml

demos/critical_filtering.py

demos/wiener_filter_via_curvature.py

demos/wiener_filter_via_hamiltonian.py

docs/source/start.rst

nifty4/__init__.py

nifty4/minimization/scipy_minimizer.py

nifty4/minimization/scipy_minimizer.py.bak

nifty4/operators/fft_operator.py

nifty4/operators/harmonic_transform_operator.py

test/test_operators/test_adjoint.py

test/test_operators/test_fft_operator.py

test/test_operators/test_harmonic_transform_operator.py

test/test_operators/test_response_operator.py