Merge branch 'master' into tests

demos/probing.py

demos/wiener_filter_hamiltonian.py

nifty/__init__.py

nifty/energies/energy.py

nifty/field.py

nifty/field_types/field_array.py

nifty/minimization/__init__.py

nifty/minimization/relaxed_newton.py

nifty/operators/fft_operator/fft_operator.py

nifty/probing/mixin_classes/__init__.py

nifty/probing/mixin_classes/diagonal_prober_mixin.py

nifty/probing/mixin_classes/mixin_base.py

nifty/probing/mixin_classes/trace_prober_mixin.py

nifty/probing/prober/prober.py

nifty/spaces/gl_space/gl_space.py