Removed the BelowUnits trait

as discussed in #3670. It is replaced by per sequence ZOffsets and won't conflict with it anymore.
It is a big problem in TD:
![screen shot 2014-10-04 at 12 09 27](https://cloud.githubusercontent.com/assets/167819/4513720/713cfe16-4b52-11e4-856d-13a5f96051d5.png)

You can fix this by adding husk sequences with `ZOffset: -512`, as you have done for fields:

```yaml
htnk.destroyed:
	idle: htnk
		Start: 0
		Facings: 32
		ZOffset: -512
	turret: htnk
		Start: 32
		Facings: 32
		ZOffset: -512
```