Fix all atoms whose position falls within a specified domain.

<p>
Usage: <tt>input = [ <i>enable  sx_min sx_max sy_min sy_max sz_min sz_max</i> ]</tt>
<br>
If <tt><i>enable</i></tt> = 1, then all atoms <tt>i</tt> whose scaled coordinates fall
within a rectangular region, i.e. 
<tt>sx_min <= SR[i].x < sx_max</tt>, 
<tt>sy_min <= SR[i].y < sy_max</tt>, 
<tt>sz_min <= SR[i].z < sz_max</tt>, will be fixed (i.e. <tt>fixed[i] = 1</tt>).

<p>
If, however, <tt><i>enable</i></tt> = 2, then all atoms whose real coordiantes fall 
within a cylindrical (tube) region will be fixed.  In this case, the <tt>input</tt> array 
will be interpreted differently.<br>
Usage: <tt>input = [ <i>enable  ind  x0 y0 z0 r_min r_max h_min h_max </i> ]</tt>
<br>
<tt><i>ind</i></tt> = 1, 2, or 3 specifies whether the cylinder is aligned along the x, y, or z
axis.  <tt><i>x0, y0, z0</i></tt> is a point (in real coordinates) on the center axis of the cylinder.  
<tt><i>r_min, r_max</i></tt> is the inner and outer radii of the cylindrical tube,
<tt><i>h_min, h_max</i></tt> specifies the length of the cylinder.

<p>
Also see <tt>fixatoms_by_pos_topol</tt>.

