Usage: <tt>input = [ <i>ng  group_1 group_2 &hellip; group_ng</i> ]  relax_by_group<tt>

<p>
If <tt><i>ng</i></tt> > 0, then all atoms <tt>i</tt> whose <tt>group[i]</tt> value 
matches any of the <tt><i>group_1 group_2 &hellip; group_ng</i> </tt> will be relaxed, while
other atoms remain fixed.

<p>
Redundant function.  Can be reproduced by <tt>fixatoms_by_group</tt> and <tt>relax</tt>.
