(Re-) Add a helper structure that holds the connectivity information for
a particular topology connection (e.g. from points to cells) to make it
easier to manage connections in multiple different directions in
CellSetExplicit.
Unlike the previous version of connectivity, this structure is
considered "internal" and not exposed through the API so that
CellSetExplicit can better manage the data. Also, many of the helper
methods remain in CellSetExplicit since they were specific for point-to-
Also, CellSetExplicit has a mechanism to take an arbitrary pair of
TopologyElementTags and get the appropriate connectivity. This should
simplify adding connections in the future.
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