add support for accessing line number from cached thermo data

src/library.cpp

@@ -773,6 +773,10 @@ argument string.
      - 1 if setup is not completed and thus thermo data invalid, 0 otherwise
      - pointer to int
      - no
+   * - line
+     - line number (0-based) of current line in current file or buffer
+     - pointer to int
+     - no
    * - step
      - timestep when the last thermo output was generated or -1
      - pointer to bigint
@@ -814,6 +818,9 @@ void *lammps_last_thermo(void *handle, const char *what, int index)
     if (strcmp(what, "setup") == 0) {
       val = (void *) &lmp->update->setupflag;
 
+    } else if (strcmp(what, "line") == 0) {
+      val = (void *) th->get_line();
+
     } else if (strcmp(what, "step") == 0) {
       val = (void *) th->get_timestep();