[Scheduling] Allow cloning of problem instances

Test whether the copy constructor and assignment operator is usable with `Problem` instances. If not, we should introduce a `clone()` method on the problem hierarchy, to be overridden by subclasses to achieve the same effect.

Inspect the Problem hierarchy and determine whether its copy constructor and assignment operator can be used for scheduling problem instances. If they are not usable, trace the subclasses and assess the clone() design described in the issue. Done means either confirming existing copying works or adding a consistent cloning approach across the hierarchy, with coverage for representative Problem subclasses.

Written by the indexing model from the issue text.