Builder methods to combine multiple .dff and .col files

As in title, if i load multiple models i would like to combine them into one bigger one
```csharp

dffA = load(modelA)
dffB = load(modelA)
dffC = load(modelA)

dffA.Append(dffB, Matrix(...)) // Append model B into model A with matrix ( position and rotation )  offset
dffA.Append(dffB, Matrix(...))
dffA.Append(dffC, Matrix(...))
```
