Accept Endpoints, directories, and files in Accessors, not just Datasets

[gjoseph92]

Not everyone might want to use iyore; a folder full of NVSPL files comes to mind here.

For the ds argument, Accessors should also accept:

• iyore Endpoint
• string path to a directory
  • In this case, just create an in-memory Dataset rooted in that directory.
  • Should the structure be endpointName: *, or should each Accessor have a built-in default pattern for its file type?
• string path to a file
  • Just read and return the data using Accessor.parse