Generalized DOFDistributor
If we extend the DOFDistributor to allow for an optional set of weights, it becomes much more powerful.
For example, such an enhanced DOFDistributor can be used to describe a line-of-sight response, so we don't need a dedicated operator for this any more.
(The complicated task of computing the appropriate mappings and weights does not go away of course, but it is now separated from the distribution operation itself, which is good.)
@ensslint, @kjako, @reimar, @pfrank, @parras, would this be a way to go forward?