Merge branch 'more_tests' into 'NIFTy_5'

.gitlab-ci.yml

nifty5/data_objects/distributed_do.py

nifty5/data_objects/numpy_do.py

test/test_field.py

test/test_minimizers.py

test/test_multi_field.py

test/test_operators/test_adjoint.py

test/test_operators/test_gradients.py

test/test_operators/test_representation.py

test/test_sugar.py