Virtual methods were used in the `MeshConnectivity` classes for the
internal ray casting system. However, using virtual methods in the
execution environment is being deprecated.
This change replaces the virtual object with an object containing a
`Variant`. The `Variant` holds one of the supported mesh connectivities
and selects the correct one at runtime rather than jumping into a
virtual method.
Marked the old versions of PrepareFor* that do not use tokens as
deprecated and moved all of the code to use the new versions that
require a token. This makes the scope of the execution object more
explicit so that it will be kept while in use and can potentially be
reclaimed afterward.
