labels: introduce framework for multiple data files

subsequent data files merge types with first

starting with support for selected atom types sections, will attach small example to PR

src/write_data.cpp

@@ -182,17 +182,15 @@ void WriteData::write(const std::string &file)
   }
 
   // proc 0 writes header, ntype-length arrays, force fields
+  // label map must come before coeffs
 
   if (me == 0) {
     header();
+    atom->lmap->write_data(fp); // NOTE: always write for now, for testing
     type_arrays();
     if (coeffflag) force_fields();
   }
 
-  // label map before Atoms, Bonds, etc.
-
-  if (me == 0) atom->lmap->write_data(fp); // NOTE: always write for now, for testing
-
   // per atom info in Atoms and Velocities sections
 
   if (natoms) atoms();