We want to be able to get topological connections where it is difficult
to know how many values you get each time. In this change, the type of
the vector holding the from indices is determined from the connectivity
object, and the worklet does not know the type (it must be templated).

Although you do not need to specify the max number for this value set
(you still currently do for field values), we still need to change the
type for explicit sets that uses something that does not rely on the Vec
class. The cell-to-point method also needs a Vec wrapper that allows it
to shorten the vector dynamically.