fixes for MPI; better type checks

demos/log_normal_wiener_filter.py

demos/nonlinear_critical_filter.py

demos/paper_demos/cartesian_wiener_filter.py

demos/wiener_filter_easy.py

demos/wiener_filter_via_curvature.py

nifty/data_objects/random.py

nifty/field.py

nifty/library/critical_power_energy.py

nifty/operators/diagonal_operator.py

nifty/operators/fft_operator.py

nifty/plotting/plot.py

nifty/sugar.py

nifty/utilities.py