Remove taxonIDReference, relax requirements for taxonID

[peterdesmet]

Closes #356

• Remove taxonomic.taxonIDReference, since it is better to include this info as part of the taxonID.

• Remove observations.taxonID

  Note: I did not update the definition of scientificName to include:

  Foreign key to package.taxonomic.scientificName.

  Since that could be confusing. The best place to indicate the relationship between observations.scientificName and package.taxonomic is in package.taxonomic, where it is already mentioned:

  Taxonomic coverage of the package, based on the unique observations.scientificName.

• Update taxonomic.taxonID:

  • Make it optional => taxonomic scope can be created from data
  • Move it after scientificName (which is required)
  • Suggest as part of the definition:

  Preferably a global unique identifier issued by an authoritative checklist.

  • Use Checklistbank URL as example

• Update example dataset to reflect these changes