Allow deselection partial classes.

[HansOlsson]

The basic issue are:

• It was a bit unclear
• Without this rule one has to introduce an intermediate level to be sure (see below), which is just weird.
• One has to check whether a class is partial, even if it is isn't used

Old work-around:

model MyAmazingClass
  extends AExtra(break b);
end MyAmazingClass;
model AExtra "Work-around"
  extends A(redeclare B b);
end AExtra;

New without work-around:

model MyAmazingClass
  extends A(break b);
end MyAmazingClass;

with:

model A
  replaceable PartialB b;
end A;
partial model PartialB end PartialB;
model B
  extends PartialB;
end B;