refactor variable info in Info class, make info accessible from library interface

tools/lammps-gui/lammpswrapper.h

@@ -35,6 +35,7 @@ public:
 
     int id_count(const char *idtype);
     int id_name(const char *idtype, int idx, char *buf, int buflen);
+    int variable_info(int idx, char *buf, int buflen);
 
     double get_thermo(const char *keyword);
     void *last_thermo(const char *keyword, int idx);