Avoid creating copy of the original nwb file at insert

[samuelbray32]

Is your feature request related to a problem? Please describe.

• Currently, sgi.insert_sessions() makes a copy of the nwb file with external link objects back to the original for large data items (e.g. electric series)
  • Was originally done to enable moving the original large file off the filesystem once intitial analysis completed
  • With fetch_nwb now able to access files on Dandi, no longer need the stand-in copy to enable moving big raw files off filesystem
• We should instead use the original nwbfile in place of the copy
• This shouldn't cause any noticable change to user's data access steps

Steps to implement

• Move the raw nwb file to be named the same as the underscore file, deleting the underscore file
• Edit the file database to update the checksum / size information for the file in the database
• Update the code to stop making the underscore copy during insert.
• Update the Import tutorial.
• Check spyglass where the naming system is assumed