rework dobj interface and prepare tests for MPI

nifty/data_objects/numpy_do.py

test/common.py

test/test_field.py

test/test_minimization/test_minimizers.py

test/test_operators/test_adoint.py

test/test_operators/test_composed_operator.py

test/test_operators/test_diagonal_operator.py

test/test_operators/test_fft_operator.py

test/test_operators/test_laplace_operator.py

test/test_operators/test_response_operator.py

test/test_operators/test_smoothing_operator.py

test/test_spaces/test_gl_space.py

test/test_spaces/test_hp_space.py

test/test_spaces/test_interface.py

test/test_spaces/test_lm_space.py

test/test_spaces/test_power_space.py

test/test_spaces/test_rg_space.py