change directory name back to nifty, but keep nifty2go package name

nifty/__init__.py

nifty/basic_arithmetics.py

nifty/domain_object.py

nifty/domain_tuple.py

nifty/energies/__init__.py

nifty/energies/energy.py

nifty/energies/line_energy.py

nifty/energies/memoization.py

nifty/energies/quadratic_energy.py

nifty/field.py

nifty/field_types/__init__.py

nifty/field_types/field_array.py

nifty/field_types/field_type.py

nifty/library/__init__.py

nifty/library/critical_filter/__init__.py

nifty/library/critical_filter/critical_power_curvature.py

nifty/library/critical_filter/critical_power_energy.py

nifty/library/log_normal_wiener_filter/__init__.py

...ormal_wiener_filter/log_normal_wiener_filter_curvature.py

...g_normal_wiener_filter/log_normal_wiener_filter_energy.py