Finding the minimum energy path and the energy barrier using the
string method.

<p>
<tt>stringrelax</tt> is more stable than <tt>nebrelax</tt> (in which we
need to choose the spring constant carefully).
However, the parallel version of <tt>nebrelax</tt> is faster than
the parallel version of <tt>stringrelax</tt> (because reparametrization
requires communication between all CPUs).

<p>
Also see <tt>nebrelax</tt>, <tt>nebspec</tt>.
