need docs + example for O2MIterator

[cyrush]

No description provided.

[cyrush]

here is an example of use in parmetis to iterate over polytopal elements:

conduit/src/libs/blueprint/conduit_blueprint_mpi_mesh_parmetis.cpp

Line 743 in 15d20c8

o2mrelation::O2MIterator o2miter(dom_topo["elements"]);

I also want to think about this in the context of ND Iteration.

Are there changes we should make to O2MIter to provide a similar interface for both cases.