Each type of point coordinates has its own class with the name
PointCoordinates*. Currently there is a PointCoordiantesArray that contains
an ArrayHandle holding the point coordinates and a PointCoordinatesUniform
that takes the standard extent, origin, and spacing for a uniform rectilinear
grid and defines point coordiantes for that. Creating new PointCoordinates
arrays is pretty easy, and we will almost definitely add more. For example,
we should have an elevation version that takes uniform coordinates for
a 2D grid and then an elevation in the third dimension. We can probably
also use a basic composite point coordinates that can build them from
other coordinates.

There is also a DynamicPointCoordinates class that polymorphically stores
an instance of a PointCoordinates class. It has a CastAndCall method that
behaves like DynamicArrayHandle; it can call a functor with an array handle
(possible implicit) that holds the point coordinates.