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.
A general purpose `CellLocator` that should work with any type of dataset.
Internally, it tries to chose one of the existing cell locators that would be
optimal for the input data.
Also removes `CellLocatorHelper`.