Method numParticles

SRC/element/PFEMElement/ParticleGroup.h:50–50

                     const VDouble& accel, double p);
    void removeParticles(const VInt& rm);
    int numParticles() const {return (int)particles.size();}
    Particle* getParticle(int i) {
        return (i>=0&&i<numParticles())? particles[i]:0;
    }