Get node by path

This is really for unit testing convenience and enables the following

````php
  $node = $qn->getNodeByPath('where.constraint.operand_dynamic[1]);
  $node = $qn->getNodeByPath('order_by.ordering[2].operand_dynamic);
````

rather than

````php
$where = $qb->getChildOfType('where');
$constraint = $where->getChildOfType('constraint');
$operands = $constraint->getChildrenOfType('operand_dynamic');
// if (!isset($operands[1] ...
$operand = $operands[1];
```