Usage: <tt>input = [ <i>enable A dx dy dz skx sky skz </i> ] makewave </tt>

<p>
Assign a displacement field to the atoms in the form of a plane wave.
<tt><i>A</i></tt> is the amplitude of the wave.
<b>dr</b> = (<tt><i>dx,dy,dz</i></tt>) is a unit vector (it is automatically normalized
at the beginning of this command).  <b>sk</b> = (<tt><i>skx,sky,skz</i></tt>) is the
wave vector for scaled coordinates.  The displacement field in real coordinates is

<p>
 <tt><b>du</b>[i] = A * <b>dr</b> * exp( <b>sk</b> &middot; <b>SR</b>[i] * 2&pi; ) </tt>

<p>
Be ware of the use of real and scaled coordinates in this expression.
Note that the velocity is not assigned, so that the configuration corresponds to
a superposition of two plane waves travelling in opposite directions.

<p>
Note that this only works for a simple crystal structure (with one atom basis).
The displacement field of a complex lattice (with more than one atom basis)
is not known a priori.
