include example use of PotentialFileReader class

to implement the behavior expected for looking up and reading/parsing

files with potential parameters in LAMMPS.  The potential file reader

class requires a LAMMPS instance, requires to be run on MPI rank 0 only,

will use the :cpp:func:`utils::get_potential_file_path` function to look up and

open the file, and will call the :cpp:class:`LAMMPS_NS::Error` class in

case of failures to read or to convert numbers, so that LAMMPS will be

aborted.

will use the :cpp:func:`LAMMPS_NS::utils::get_potential_file_path`

function to look up and open the file, and will call the

:cpp:class:`LAMMPS_NS::Error` class in case of failures to read or to

convert numbers, so that LAMMPS will be aborted.

.. code-block:: C++

   :caption: Use of PotentialFileReader class in pair style coul/streitz

    PotentialFileReader reader(lmp, file, "coul/streitz");

    char * line;

    while((line = reader.next_line(NPARAMS_PER_LINE))) {

      try {

        ValueTokenizer values(line);

        std::string iname = values.next_string();

        int ielement;

        for (ielement = 0; ielement < nelements; ielement++)

          if (iname == elements[ielement]) break;

        if (nparams == maxparam) {

          maxparam += DELTA;

          params = (Param *) memory->srealloc(params,maxparam*sizeof(Param),

                                              "pair:params");

        }

        params[nparams].ielement = ielement;

        params[nparams].chi = values.next_double();

        params[nparams].eta = values.next_double();

        params[nparams].gamma = values.next_double();

        params[nparams].zeta = values.next_double();

        params[nparams].zcore = values.next_double();

      } catch (TokenizerException & e) {

        error->one(FLERR, e.what());

      }

      nparams++;

    }

A file that would be parsed by the reader code fragment looks like this:

   # DATE: 2015-02-19 UNITS: metal CONTRIBUTOR: Ray Shan CITATION: Streitz and Mintmire, Phys Rev B, 50, 11996-12003 (1994)

   #

   # X (eV)                J (eV)          gamma (1/\AA)   zeta (1/\AA)    Z (e)

   Al      0.000000        10.328655       0.000000        0.968438        0.763905

   O       5.484763        14.035715       0.000000        2.143957        0.000000

----------