Creates an instance from a given soup tag.

def from_soup_tag(tag):
        "Creates an instance from a given soup tag."
        sections = [Section.from_soup_tag(s) for s in tag.findAll('section')]
        return Course(
            tag['name'], tag['dept'], int(tag['num']), tag['credmin'],
            tag['credmax'], tag['gradetype'], [s for s in sections if s.is_valid]
        )