…h type attributes

- A change to adopt the same logic as file objects
  regarding the different hash values
- In STIX 2.1 an Artifact object is not necessarily
  linked to a File object and both referenced by
  an Observed Data object. In some cases Artifact
  objects are referenced for instance by Malware
  objects, in which case they describe the actual
  malware sample. It is then usefull to have the
  different hash values in single attributes rather
  than concatenated in a text attribute