Signal._deepcopy_with_new_data does not copy Signal._plot anymore
Signal.get_current_signal is now faster
BUG-FIX: the navigator image was wrong when navigation_dimension > 2
BUG-FIX: Signal.plot was raising an exception when signal_dimension == 2 & navigation_dimension > 2
Rename AxesManager._indices_generator -> _array_indices_generator, add _am_indices_generator, _navigation_shape_in_array, _signal_shape_in_array
Fixes #83 and related issues