update mesh utils iterator to support shape_map style mixed element meshes

[cyrush]

Old stream id concept is now deprecated, so update the ele iterator to support shape maps style meshes:

conduit/src/libs/blueprint/conduit_blueprint_mesh_utils_iterate_elements.hpp

Line 220 in c990a81

else

Note: this code underpins the partitioning

[mlohry]

I just found myself looking for this feature. I have mixed element meshes of hex/tet/pyramid/wedge, and partitioning this does appear to work, where it does work for a plain all-tet mesh.