Clean up CellSet

Underneath the CellSet implementation is a set of supporting classes that manage the actual structure in both the control and execution environments. However, the implementation of these classes was a bit confusing and inconsistent. The following changes are made:

• Most significantly, there is no longer any Connectivity classes in the control environment. This functionality has been wrapped up into the CellSet classes, which is more consistent and easier to understand. (There was a definite distinction between CellSet and Connectivity, but it was subtle and difficult to understand.) This also means that edits to CellSets happen to CellSets directly.

• The set of classes for structured and explicit cell sets match. There is different functionality within, but the class naming and meaning are consistent.

• Make the class names more consistent with the rest of VTK-m class names. Specifically classes like ExplicitConnectivity become ConnectivityExplicit. Also, the words regular and structured were being used interchangeably. Now, always use structured except when dealing specifically with grids of regular spacing.

• The connectivity classes were using the nomenclature "From" and "To" to specify topological elements of links. The same concept in worklet classes were using the nomenclature "Src" and "Dest." For consistency, all references are changed to "From" and "To".

• Unlike explicit cell sets, structured cell sets have functionality shared between control and execution environments. Rather than duplicate it or create unique exposed classes, have a shared internal implementation in vtkm::internal.