Merge branch 'NIFTy_5' into more_tests

Dockerfile

docs/source/conf.py

docs/source/ift.rst

nifty5/__init__.py

nifty5/extra.py

nifty5/field.py

nifty5/library/adjust_variances.py

nifty5/library/correlated_fields.py

nifty5/library/inverse_gamma_operator.py

nifty5/library/smooth_linear_amplitude.py

nifty5/minimization/metric_gaussian_kl.py

nifty5/minimization/scipy_minimizer.py

nifty5/operators/contraction_operator.py

nifty5/operators/domain_tuple_field_inserter.py

nifty5/operators/endomorphic_operator.py

nifty5/operators/energy_operators.py

nifty5/operators/linear_interpolation.py

nifty5/operators/sampling_enabler.py

nifty5/operators/simple_linear_operators.py

nifty5/plot.py