A couple of new routines to query usage of children

The type timer_t got two new kind of methods to query information about
the resource usage if its children. The method

  %get_in_children

queries the seconds spent in all children of the queried node, and

  %get_without_children

conversely returns the number of seconds spent excluding all children.

Additionally, there also version where the node in question is passed
directly instead of by a string path (*_node) and versions where the
result is a type(value_t) with all measurements (get_value_*).