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.
`PointLocator`s have changed from being virtual objects to being trivial
objects. Part of this change means that when a worklet gets the
execution object for a point locator, it gets the actual object (or a
reference to it) instead of a pointer to an object. This means that the
new code uses the `.` operator to access the locator's features instead
of the `->` operator.
To support code still using the deprecated functionality, added a
specific `->` operator to the locator execution object to make it behave
as if it were a pointer. However, this operator is marked deprecated to
warn the user that they should modify their code to use the `.` instead.
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.)
