Rather than try to collect all `LastCell` types inside of a single
header and make an uber type, have each cell locator define its own cell
locator type and use that.
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.)
With recent changes to `ArrayHandle`, the type for the associated array
portal is now the same across all devices. This means that almost all
exec objects no longer need to be specialized on the device types. Thus,
clean up the locator exec objects to no longer need to be templated on
device.
