Base::addFilter

public function addFilter($key, $value)
{
	if ($this->isAvailableFilter($key))
	{
		$this->filters[$key][] = $this->normaliseFilterValue($value);
		return true;
	}
	else
	{
		$this->errors[] = new Error('Filter with key:' . $key . ' not available for this provider');
		return false;
	}
}