message: Fixed warnings about comparison across signed/unsigned
diff: @@ -629,7 +629,7 @@ std::string Molecule::formula(const std::string& delimiter, int over) const
  iter = composition.find(6);
  if (iter != composition.end()) {
  result << "C";
  - if (iter->second > over)
  + if (iter->second > static_cast<size_t>(over))
  result << delimiter << iter->second;
  composition.erase(iter);
  @@ -637,7 +637,7 @@ std::string Molecule::formula(const std::string& delimiter, int over) const
  iter = composition.find(1);
  if (iter != composition.end()) {
  result << delimiter << "H";
  - if (iter->second > over)
  + if (iter->second > static_cast<size_t>(over))
  result << delimiter << iter->second;
  composition.erase(iter);
  }
  @@ -647,7 +647,7 @@ std::string Molecule::formula(const std::string& delimiter, int over) const
  iter = composition.begin();
  while (iter != composition.end()) {
  result << delimiter << Elements::symbol(iter->first);
  - if (iter->second > over)
  + if (iter->second > static_cast<size_t>(over))
  result << delimiter << iter->second;
  ++iter;
  }
  