Model: add process_row limiter

[mihaivarga]

$this->models->example->process(3)->get($id)
$this->models->example->process(3)->get_all

Should run process row only 3 levels down

$this->models->example->process(0)->get($id)

Should not run process_row

By default general process level should be manageable from $page->models_process_level=10
You should be able to set a default process level for each model $this->process_level=10

$this->process(3) will set the framework process level to 3 after the process level it's reached it should reset the process level for each model to it's default one.