Incremental index by name in Append

[Jorge]

It would be desirable if after doing SetIndexByName on a Set the following invocation to does not invalidate the index Append and keep the Set indexed.

[Víctor de Buen Remiro]

Current behaviour is incoherent due Append does no remove the index nor increment it. In this example, after the call to Append function HasIndexByName returns True but it cannot find the new element.

  Set s = {[[
   Real a = 1;
   Real b = 2;
   Real c = 3;
   Real d = 4;
   Real e = 5;
   Real f = 6 ]]};

  Real ok.1 = SetIndexByName(s);
  Real ok.idx.1 = FindIndexByName(s,"c")==3;

  Set Append(s, {[[ Real g = 7 ]]});
  Real ok.2 = HasIndexByName(s);
  Real ok.idx.2 = FindIndexByName(s,"g")==7;

[Víctor de Buen Remiro]

(In [1600]) Fixes #773

[Víctor de Buen Remiro]

There is a new optiaonal argument

  Set Append(Set set1, Set set2, [Real incrementalIndexByName=False])

If incrementalIndexByName is true, then the index of names will be incremented if original set had an index and new elements are compatible with old ones; or created if set was null.

If incrementalIndexByNameis false and set had an index, then it will be removed.