Parallelize: create decay cache

[ferdymercury]

It could be quite useful to parallelize this part with openMP with proper mutexes:

  for (auto &a : aList)
  {
    NuclideId na;
    na.set_A(a);
    ENSDFTreeItem *aa = new ENSDFTreeItem(ENSDFTreeItem::DaughterType,
                                          na,
                                          QList<QVariant>() << ("A=" + QString::number(a)),
                                          true,
                                          root);