break GasPropertiesModel into smaller pieces

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Of the 123 .js files in this sim, all but 5 are less than 300 lines, and they are:

325 ./common/view/GasPropertiesScreenView.js
362 ./diffusion/model/DiffusionModel.js
377 ./common/model/CollisionDetector.js
557 ./common/view/GasPropertiesIconFactory.js
692 ./common/model/GasPropertiesModel.js

I'm please with all of these except GasPropertiesModel.js. It's not huge, but it's a lot to digest in one "chunk". So I had the thought that I might break it up into smaller "sub models" related to PV = NkT. I started exploring this in my working copy, and it looks promising. My current thought is:

ParticleSystem - responsible for number of particles (N) and managing particles
PressureModel - responsible for pressure (P)
TemperatureModel - responsible for temperature (T)
BaseContainer - already exists, add responsibility for volume (V)

@veillette FYI. I don't think this is reason to delay the preliminary code review #109. But I may be working on GasPropertiesModel, and it may get divided up a bit.

[pixelzoom]

This is done. GasPropertiesModel has been broken up as described in #112 (comment). I'm much happier with this. It's more modular, easier to understand, and should be more maintainable.

I reviewed DiffusionModel to see if something similar would be useful there. But it's a small model (362 lines), much less complicated, and I think it's small enough to digest "as is".

@veillette FYI, this is in good shape for #109.

Closing.