In #291 we change the values and variances properties to return py::array_t.

• This is not possible for non-POD types (std::string, Dataset, boost::small_vector, ...).
• In the current implementation we are still returning the flattened VariableView (or span) in those cases. This is inconsistent and should be changed to return something equivalent to a numpy.ndarray (in terms of the basis interface provided, such as shape and index access).
• Doing so might be much easier if we also make this change on the C++ side, i.e., provide a shaped alternative to VariableView. However, this may be tricky since the view type might depend on the number of dimensions (since it affects, e.g., the return type of operator[]), so it is not clear whether this is a desirable change on the C++ side.
• Once this is complete, change the dims property to match the behavior of that in xarray.DataArray, i.e., include only the labels, not the shape (done, without underlying C++ change).