Revise mixin structure to reflect table types

[CBroz1]

The mixin class has expanded to include a lot of features

• Long distance restrictions
• thread checking
• adjusted populate
• fetching nwb
• deletes
  • delete downstream parts (no longer needed after DJ 14.2)
  • check delete permissions
• Exports

Moving forward, this might be easier to navigate if broken into classes reflecting our conceptual table types: data, selection, params.

From there, we could add methods like insert_default to SpyglassParams, which would inherit both SpylgassMixin and dj.Lookup.

Possible features...

• insert_default for params
• insert_with_default for selection
• ...