Merge remote-tracking branch 'origin/NIFTy_4' into addUnits

.gitlab-ci.yml

ci/install_basics.sh

demos/critical_filtering.py

demos/wiener_filter_via_curvature.py

demos/wiener_filter_via_hamiltonian.py

docs/source/start.rst

nifty4/__init__.py

nifty4/data_objects/distributed_do.py

nifty4/data_objects/numpy_do.py

nifty4/dobj.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_minimization/test_minimizers.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