`selectType([...])`: introduce changes to allow operator on references with `[0..1]` multiplicities

[maxkratz]

Currently, the selectType([...]) operator can only be used on collections.

It would be nice to have it adapted such that it can also be used on references with [0..1] multiplicities.

context.nodes.w.next.next->selectType(Workload)->sum(context.value * element.minNurseSkill)
// alternative
context.value * ((Workload) context.nodes.w.next.next).minNurseSkill

Example was taken from: https://github.com/Echtzeitsysteme/gips-examples/blob/c2b98e3741ea9b0349d0e9b3a84b002740e3bbc4/ihtcvirtualgipssolution/src/ihtcvirtualgipssolution/Model.gipsl#L698