#988 Add support for automatic parenting and inheritance

[David P. Chassin]

The VADER team would like to have the following capabilities added.

Have the loader support automatic find of next childless object to connect a parent, e.g.,

object triplex_meter {
    parent childless:groupid=secondary;
    // ...
}

This requires adding code to object_properties() as well as resolve_object(), but it's pretty simple. It would also require adding a child_count property to the object header to facilitate finding childless objects, with a corresponding increment in object_set_parent().

As for the second feature, this would require adding support for an inherit special value to object_properties() so that the following would be allowed:

object triplex_meter {
  phases inherit;
}

In the cases of phases specifically, we can alternatively simply have phases be copied from the parent when it's not defined, as we do with nominal_voltage.