Usage: <tt>input = [ <i>x0 y0 z0 r</i> ] printatoms_in_sphere</tt>

<p>
Print the indices of all atoms whose real coordinates fall within a sphere
centered at <tt><i>x0 y0 z0</i></tt> with radius <tt><i>r</i></tt>.  These
atoms can then be fixed by <tt>fixatoms_by_ID</tt>.

