Fix mappings in nnpdfsettings parsing

Created by: Zaharid

In nnpdfsettings.h, we have mappings like:

  map<int,DataSetInfo>     fDataSetInfo;  //!< Contains the dataset info
  map<int, PosSetInfo>     fPosSetInfo;   //!< Map of PosSetInfo structs

which are constructed like:

          // Generate hash of setname
          std::hash<std::string> str_hash;
          const size_t hashval = str_hash(setname);

          DataSetInfo info = {setname, setsys, setfrac, cfactors};
          map<int,DataSetInfo>::const_iterator iMap = fDataSetInfo.find(hashval);
          if (iMap != fDataSetInfo.end()) { cerr << Colour::FG_RED << "NNPDFSettings::LoadExperiments error: hash collision for set: " << setname << Colour::FG_DEFAULT << endl; exit(-1); }
          else { fDataSetInfo.insert(make_pair(hashval, info)); }
          nsetname.push_back(setname);
          fSetName.push_back(setname);

which is a bit silly. It overcomplicated, as we can just as well have map<string, Type>, and we don't need to check for hash collisions, since maps can work anyway.