xarray methods additions

[FabianHofmann]

From release notes:

• The Constraint class now supports the methods assign, assign_attrs, assign_coords, broadcast_like, chunk, drop_sel, drop_isel, expand_dims, sel, isel, shift, swap_dims, set_index, reindex, reindex_like, rename, rename_dims, roll, stack. These methods allow to manipulation of a (anonymous) constraint more flexibly.

• The Variable, expressions and Constraint classes now have new methods swap_dims and set_index. The swap_dims method allows to swap the dimensions of the object. The set_index method allows to set a new index for the object. Both methods are useful for reshaping the object more flexibly.