Consider unifying local and instantiation metadata for local datafiles/datasets

[cortadocodes]

This would let us move to more intuitive cloud metadata updating such as

x.upload(metadata_only=True)

instead of

x.update_metadata()

More info

• It would also reduce possible metadata locations to just:
  • Cloud
  • Local
• Providing metadata during instantiation of a local datafile would ovewrite any locally stored metadata for that file.
• Providing metadata during cloud datafile instantiation would still be ignored