input: Write a python function to filter links that have not been visited and start with the given domain.
  assert filter_links(self, [Link("http://example.com/page1"), Link("http://example.com/page2"), Link("http://other.com/page")])==[Link("http://example.com/page1"), Link("http://example.com/page2")]
  assert filter_links(self, [Link("http://example.com/page3"), Link("http://example.com/page4")])==[Link("http://example.com/page3"), Link("http://example.com/page4")]
  assert filter_links(self, [Link("http://other.com/page1"), Link("http://other.com/page2")])==[]
output: def filter_links(self, links):
      domain = "http://example.com"
      return [link for link in links if link.url.startswith(domain) and not link.visited]