There was some compile errors with CUDA and the CellLocatorGeneral.
Apparently it added enough to the CUDA kernel to cause issues with
constant memory.
Deprecated the `CellLocator` class and made all methods of the
other `CellLocator` classes non-virtual. General locators can
still use the `CellLocatorGeneral` class, but this class now
only works with a predefined set of locators. (The functionality
to provide a function to select a locator has been removed.)
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`.
