Merge branch 'feature/field_multiple_space' into smooth_operator

nifty/__init__.py

nifty/minimization/__init__.py

nifty/minimization/conjugate_gradient.py

nifty/operators/__init__.py

nifty/operators/diagonal_operator/diagonal_operator.py

nifty/operators/fft_operator/fft_operator.py

nifty/operators/linear_operator/linear_operator.py

nifty/operators/propagator_operator/__init__.py

nifty/operators/propagator_operator/propagator_operator.py