Rewrite the function get_group_wkt() using geoPHP

Now that this geoPHP bug has been fixed, we can replace the function get_group_wkt() with:

  /**
   * Combine all group wkt into a single geometry collection
   *
   * openlayers_views_style_data specific
   */
  function get_group_wkt($wkt_array) {
    geophp_load();
    $geophp = geoPHP::load($wkt_array, 'wkt');
    return $geophp->out('wkt');
  }

I'm looking for people to test and give me feedback. The patch is following.