Remove 'System' from name 'ParticleSystem', named it to 'Particles'.
Rename 'get/setNumofParticles' to 'get/setMaxParticles'.
Remove the use of Physics::Computation in ParticlesManager

ParticlesManager will probably have its own computations instead of
using one from Physics.
Allow compiler generated copy constructor and assignment operator.
Update CMakeLists.txt for Particles' UnitTest
Use forward declaration in ParticlesManager.h, remove unused headers
Add Particle data struct
Revise Particles::Representation, update UnitTests.
Add ParticleEmitter