Finished refactoring of probing classes. Now uses mixin-classes.

nifty/nifty_utilities.py

nifty/operators/fft_operator/__init__.py

nifty/operators/linear_operator/linear_operator.py

nifty/operators/probing_operator/diagonal_prober.py

nifty/operators/probing_operator/trace_prober.py

nifty/probing/__init__.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/__init__.py

nifty/probing/prober/prober.py

nifty/spaces/space/space.py