fix: [stix import] Using subclasses + Support of STIX coming from multiple MISP events
fix: [stix import] Using subclasses + Support of STIX coming from multiple MISP events

- Using subclasses here allowed us to simplify some
  variables declarations and to reuse some functions
- STIX coming from multiple MISP events refers to
  STIX files created via the MISP to STIX export
  that can include multiple MISP events

devnull-/MISP,devnull-/MISP,Rafiot/MISP,FIRSTdotorg/MISP,FIRSTdotorg/MISP,MISP/MISP,MISP/MISP,FloatingGhost/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,FloatingGhost/MISP,SteveClement/MISP,FIRSTdotorg/MISP,MISP/MISP,FloatingGhost/MISP,SteveClement/MISP,devnull-/MISP,Rafiot/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,Rafiot/MISP,devnull-/MISP,Rafiot/MISP