Rename 'Particle::PartileManager' to 'Particle::Manager'.
Rename 'Particles::ParticleEmitter' to 'Particle::Emitter'.
Remove 'addParticle(s)' from Particles::Representation's API.

They will be APIs of Particles.
Rename struct 'Particle' to 'ParticleData'.
Rename 'Particles::Particle' to 'Particles::Particles'.
New class 'Particles' subclassing Vertices<ParticleData>

'Particles' is a SurgSim::DataStructures::Vertices<> with specialized
template parameter 'ParticleData'.

It has two data members. One defines the maximum amount of particles
allowed and another defines the lifetime of all the particles.

In 'Particles::doUpdate()', it will update the ages of all the active
particles, those who exceed 'm_lifeTime' will be marked as 'invalid'.