trtAssign return integer or factor

[elinsooon]

fix trtAssign to return type factor (or integer)

[elinsooon]

is it sufficient for trtAssign to just return type integers? Once it's in this form it can be easily coerced to type factor, correct? If it is sufficient, we can close this issue

[kgoldfeld]

I don't think it would be a great idea to force it to be a factor at this point, because it might mess with some existing user code, but we might want to consider adding an explicit argument to create a factor (e.g. asFactor). So, maybe we can keep it open for now in case we decide to do that.