Added default_spaces property to Operator classes.

demos/wiener_filter_hamiltonian.py

nifty/nifty_utilities.py

nifty/operators/diagonal_operator/diagonal_operator.py

nifty/operators/fft_operator/fft_operator.py

...rs/invertible_operator_mixin/invertible_operator_mixin.py

nifty/operators/linear_operator/linear_operator.py

nifty/operators/projection_operator/projection_operator.py

nifty/operators/propagator_operator/propagator_operator.py

nifty/operators/smoothing_operator/smoothing_operator.py