merge NIFTy_5; only keep important changes

.gitlab-ci.yml

Dockerfile

README.md

demos/bernoulli_demo.py

demos/getting_started_1.py

demos/getting_started_2.py

demos/getting_started_3.py

demos/plot_test.py

demos/polynomial_fit.py

docs/source/code.rst

docs/source/conf.py

docs/source/ift.rst

docs/source/images/getting_started_3_results.png

docs/source/images/getting_started_3_setup.png

docs/source/installation.rst

nifty5/__init__.py

nifty5/compat.py

nifty5/data_objects/distributed_do.py

nifty5/data_objects/numpy_do.py

nifty5/data_objects/random.py