Single sections support

This is is feature request for single sections. It introduces a new method <code>subsection</code> in <code>tree.py</code> which is similar to <code>subsections</code> method. However, it does not return a list but a single <code>CfgSection</code>. It is only allowed to be called if there is exactly one section of this type present in the configuration, so schema has <code>_meta = {'repeat': (1,1)}</code>. Otherwise, an error is raised.