Make it faster  and stricter 
The current validation is unevenly distributed over the parse_swhid function
and the SWHID class . Keeping the validation strictly within the scope of the
class sounds more in adequation with other model objects we use in the swh.model.
Note that is has been proposed to:
- Keep a reference of the core swhid when the swhid passed as parameter is a swhid with qualifiers.
- Encode the path qualifier value as bytes (if any)
 IRC discussion