Cacher is the code snippet organizer for pro developers

// to add at your plugin function file
  function adding_page_template ($templates)
    {
      $trainingTimer = WP_PLUGIN_DIR . '/myrules/3rdParty/trainingTimer/trainingTimer.php';
      $templates[$trainingTimer] = 'trainingTimer';
      return $templates;
    }
  add_filter ('theme_page_templates', 'adding_page_template');

  function redirect_page_template ($template)
    {
      global $wp_query;

      $template = get_post_meta($wp_query->post->ID,'_wp_page_template',true);

      if (basename($template) === 'trainingTimer.php') {
        return ($template)?$template:'Error loading the template';
      }
    }
  add_filter ('page_template', 'redirect_page_template');