weight() -> dvol() for domain objects

nifty/domain_object.py

nifty/field.py

nifty/field_types/field_type.py

nifty/operators/fft_operator/fft_operator_support.py

nifty/spaces/gl_space/gl_space.py

nifty/spaces/hp_space/hp_space.py

nifty/spaces/lm_space/lm_space.py

nifty/spaces/power_space/power_space.py

nifty/spaces/rg_space/rg_space.py

test/test_spaces/test_gl_space.py

test/test_spaces/test_hp_space.py

test/test_spaces/test_lm_space.py

test/test_spaces/test_power_space.py

test/test_spaces/test_rg_space.py