Hide Particle class members

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.

Fixes #737 (closed)