improved-examples (part 2)

[VladimirAlexiev]

@CraigRe thanks for accepting the prev PR so quickly! Please also accept this one

• Salvage all improvements from #285 except "Convert all XML to JSON":
  • Ontology/README.md, merge WithSensorData/README.md, merge AssociationEvent/README.md
  • SensorDataExample17.jsonld is gross/net/tare
• Remove prefix gs1:MT- from epcis-context-simple.jsonld
• add exception to epcis-context-simple.jsonld
• Add folder WithFullCombinationOfFields (from XML).
• Add local rail: context to Example-TransactionEvents-2020_07_03y.jsonld

All Turtles convert ok and look ok. But I need to reprocess them after this is accepted:

Remove prefix gs1:MT-