The member variables of the `vtkm::Particle` classes are now hidden. This
means that external code will not be directly able to access member
variables like `Pos`, `Time`, and `ID`. Instead, these need to be retrieved
and changed through accessor methods.
This follows standard C++ principles. It also helps us future-proof the
classes. It means that we can provide subclasses or alternate forms of
`Particle` that operate differently. It also makes it possible to change
interfaces while maintaining a deprecated interface.
The `ArrayCopy` method has been changed to be precompiled. It handles
most standard array types. But there are some special `ArrayHandle`
types that are not correctly handled, and these go to a slow fallback.
Find places in the code that use that fallback and fix them.
There are also some instances of replacing an `ArrayHandleCounting` with
an `ArrayHandleIndex`. This change is probably not strictly necessary to
make the `ArrayCopy` faster, but when it can be used `ArrayHandleIndex`
is generally better.
