Added third order, added documentation, took out extraneous lines, added documentation figures.

[Description:]

Calculate the dynamical matrix of the selected group.

Calculate the dynamical matrix by finite difference of the selected group,

[Restrictions:]

:c,image(JPG/dynamical_matrix_dynmat.jpg)

where D is the dynamical matrix and Phi is the force constant matrix defined by

:c,image(JPG/dynamical_matrix_force_constant.jpg).

The output for the dynamical matrix is printed three elements at a time. The

three elements are the three beta elements for a respective i/alpha/j combination. 

Each line is printed in order of j increasing first, alpha second, and i last.

The command collects the entire dynamical matrix a single MPI rank,

so the memory requirements can be very significant for large systems.

If the style eskm is selected, the dynamical matrix will be in units of inverse squared

femtoseconds. These units will then conveniently leave frequencies in THz, where

frequencies, represented as omega, can be calculated from

:c, image(Eqs/dynamical_matrix_phonons.jpg)

[Restrictions:]

This command assumes a periodic system.

The command collects an array of nine times the number of atoms in a group

on every single MPI rank, so the memory requirements can be very significant

for large systems.

This command is part of the USER-PHONON package.  It is only enabled if

LAMMPS was built with that package.  See the "Build