Refactoring of the initialization code of the 'particleMesh' variable.

[capdevon]

switch (meshType) {
    case Point:
        particleMesh = new ParticlePointMesh();
        this.setMesh(particleMesh);
        break;
    case Triangle:
        particleMesh = new ParticleTriMesh();
        this.setMesh(particleMesh);
        break;
    default:
        throw new IllegalStateException("Unrecognized particle type: " + meshType);
}

This piece of code is repeated 3 times in ParticleEmitter class, violating the DRY programming rule. So I encapsulated it in a separate method.

Fixed typo compatability -> compatibility

[stephengold]

This refactoring seems worthwhile.

[stephengold]

Thanks for your contribution to our project, @capdevon!