Implement threaded for-loops in MetaModelica

[Per Östlund]

We currently have threaded reductions, it would be nice to have the equivalent for-loops also (syntax is only a suggestion):

for threaded e1 in list1, e2 in list2 loop
  // Iterates simultaneously over list1 and list2.
end for;

=>

class M
  Real r(start = 0.0);
algorithm
  for i in 1:20 loop
    for j in 1:30 loop
      r := r + /*Real*/(i) + /*Real*/(j);
    end for;
  end for;
end M;

[Martin Sjölund]

The for-loops are currently translated into nested loops, making this require substantial changes to the DAE

model M
  Real r(start=0);
algorithm
  for i in 1:20, j in 1:30 loop
    r := r + i + j;
  end for;
end M;

=>

class M
  Real r(start = 0.0);
algorithm
  for i in 1:20 loop
    for j in 1:30 loop
      r := r + /*Real*/(i) + /*Real*/(j);
    end for;
  end for;
end M;