Add core functionality for using Elink 2.0 python client

[tsmathis]

Update MPCite to use new Elink python client from OSTI: elinkapi

Should include functionality for:

• Updating existing OSTI records
• Record new records (mint new DOIs)
• Check status of new record requests
• Delete records

Some nice to haves:

• Document models for use in tracking metadata
• Templates for common record types for ease of submission
  • could be configurable by the user

[tsmathis]

closed by #28