Add downcast helper script / function / class

[kpvdr]

There are many places in the code where a child class is known to exist, but we have only the parent pointer. Currently this is handled as follows:

Parent* parentPtr = ...;
Child* ChildPtr = dynamic_cast<Child*>(parentPtr);
if (ChildPtr == nullptr)  {
    throw Error(...);
}
\* use childPtr *\

This pattern is used many times, it might help simplify the code and make it easier to read if there were a simple helper to do all this. One possibility:

Parent* parentPtr = ...;
Child* childPtr = cast_and_check<Child*>(parentPtr);
\* use childPtr *\