Made SGPropertyNode::getPath extremely efficient: it now caches its

Made SGPropertyNode::getPath extremely efficient: it now caches its
result, so after the first call for any node, it simply tests and
returns a pointer.  This also fixes the problem of buffer conflicts
with make_string.

Added SGPropertyNode::hasChild(const char * name, int index = 0) to
provide a syntactically-cleaner way to test for the existence of a
child node, i.e.

  for (int i = 0; i < 9; i++) {
    if (node->hasChild("foo", i))
      foo[i] = node->getChild("foo", i)->getDoubleValue();
  }