write out and check for unit settings marker in data files

src/read_data.cpp

@@ -1154,6 +1154,14 @@ void ReadData::header(int firstpass)
   if (me == 0) {
     char *eof = utils::fgets_trunc(line, MAXLINE, fp);
     if (eof == nullptr) error->one(FLERR, "Unexpected end of data file");
+
+    // check for units keyword in first line and print warning on mismatch
+    auto units = Tokenizer(utils::strfind(line, "units = \\w+")).as_vector();
+    if (units.size() > 2) {
+      if (units[2] != update->unit_style)
+        error->warning(FLERR, "Inconsistent units in data file: current = {}, data file = {}",
+                       update->unit_style, units[2]);
+    }
   }
 
   while (true) {

src/write_data.cpp

@@ -245,8 +245,8 @@ void WriteData::write(const std::string &file)
 
 void WriteData::header()
 {
-  fmt::print(fp,"LAMMPS data file via write_data, version {}, "
-             "timestep = {}\n\n",lmp->version,update->ntimestep);
+  fmt::print(fp,"LAMMPS data file via write_data, version {}, timestep = {}, units = {}\n\n",
+             lmp->version, update->ntimestep, update->unit_style);
 
   fmt::print(fp,"{} atoms\n{} atom types\n",atom->natoms,atom->ntypes);