def add_link(s):
    if s.startswith('http'):
        a = '<a href="{0}" target="_blank">'.format(s)
        a += s
        a += '</a>'
        return a
if `s` is a url, then adds anchor tags for html representation in ipynb.
dataset/ETHPy150Open inpho/vsm/vsm/extensions/htrc.py/add_link