add properties example

==============================================

The library interface allows to extract different kinds of information

about the active simulation instance and also to modify some of them.

This allows to combine MD simulation steps with other processing and

about the active simulation instance and also to modify some of it.

This enables combining MD simulation steps with other processing and

simulation methods computed in the calling code or another code that is

coupled to LAMMPS via the library interface.  In some cases the data

returned is direct reference to the original data inside LAMMPS cast

can change sizes and order at every neighbor list rebuild and atom sort

event as atoms are migrating between sub-domains.

.. code-block:: C

   #include "library.h"

   #include <stdio.h>

   int main(int argc, char **argv)

   {

     void *handle;

     int i;

     handle = lammps_open_no_mpi(0, NULL, NULL);

     lammps_file(handle,"in.sysinit");

     printf("running simulation with %g atoms\n",

            lammps_get_natoms(handle));

     lammps_command(handle,"run 1000 post no");

     for (i=0; i < 10; ++i) {

       lammps_command(handle,"run 100 pre no post no");

       printf("PE = %g\nKE = %g\n",

              lammps_get_thermo(handle,"pe"),

              lammps_get_thermo(handle,"ke"));

     }

     lammps_close(handle);

     return 0;

   }

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.. doxygenfunction:: lammps_version