Usage: <tt>input = [ <i>i_dir  j_dir</i> ] switchindex</tt> 

<p>
Switch the two indices for every atom's scaled coordinates. <tt><i>i_dir, j_dir</i></tt>
= 1, 2, 3 for x, y, z.

Vector arrays <tt>SR, R, R0, VSR, VR</tt> and box matrix <tt>H</tt>
are changed.

<p>
WARNING: There seems to be a bug.  The real coordinate should not change
if both scaled coordinate and H matrix have changed.

