Add modules and TS file for GPUParticleSystem

[looeee]

GPUParticleSystem extends Object3D so I've moved it into the js/objects/ directory.

[mrdoob]

Thanks!

[xow]

Thanks heaps for including this!

I noticed it has a dependency on
https://github.com/mrdoob/three.js/blob/master/examples/textures/particle2.png
and https://github.com/mrdoob/three.js/blob/master/examples/textures/perlin-512.png not sure if that was intentional

[Mugen87]

This dependency is only optional since the respective texture parameters (particleNoiseTex and particleSpriteTex ) are optional, too. Nevertheless, I think it's cleaner to make these parameters mandatory and remove both hard coded URLs from the JS code. Do you want to make this change?