add button to recenter the image on the currently selected group's COM

tools/lammps-gui/imageviewer.h

@@ -55,6 +55,7 @@ private slots:
     void do_rot_right();
     void do_rot_up();
     void do_rot_down();
+    void do_recenter();
     void cmd_to_clipboard();
     void change_group(int);
 
@@ -90,6 +91,7 @@ private:
     int xsize, ysize;
     int hrot, vrot;
     double zoom, vdwfactor;
+    double xcenter, ycenter, zcenter;
     bool showbox, showaxes, antialias, usessao, useelements, usediameter, usesigma;
 };
 #endif