Type inference assigns boxed type to calculated attribute

The type of a calculated attribute, if not explicitly specified, is inferred from the Xbase expression describing the calculation. If the result of the Xbase expression is a boxed type, we should probably assign the unboxed type to the attribute.

Example:
```
from Film films
group by films.year
create Summary {
    year := films.same[ it.year ] // EIntegerObject instead of EInt
}
```