do we need a more robust centroid algo?

[cyrush]

for many polygon cases -- we may need a more robust centroid algo, such as:

https://en.wikipedia.org/wiki/Centroid#Of_a_polygon

I think we are using simple weighting:

https://github.com/LLNL/conduit/blob/742a29989b40ca50c70e9327a5887345e425786b/src/libs/blueprint/conduit_blueprint_mesh.cpp#L1292

We can also simplify the existing code using the new element lambdas and value accessors.