Bugfix/whitlock/performance improvements (#1068)

* Move O2MIterator creation outside loop.

* Good speed up for O2MIterator.

* Performance improvement 1 for calculate_unstructured_centroids.

* calculate_unstructured_centroids improvement 2.

* More optimizations for calculate_unstructured_centroids.

* TopologyMetadata improve populating entity_index_bag.

* TopologyMetadata eliminate a call to map::find.

* Convert a std::set to std::vector.

* TopologyMetadata - selectively skip global_id computation.

* TopologyMetadata - simplify dim_geassocs_maps, dim_leassocs_maps some.

* TopologicalMetadata - change assoc map structure a bit, improve code readability, preallocate some memory to reduce reallocations.

* Further performance improvements for calculate_unstructured_centroids.