Ensure merge tables declared in new database

[samuelbray32]

Description

Fixes #1204, #872

• Merge tables not guaranteed to be declared before imported by their parent_source make call
  • Can cause error in fresh database when tries to declare within the make transaction
  • Done to avoid circular imports
• Add new util declare_all_merge_tables which imports all core spyglass merge tables to ensure declaration
• Run during populate_all_common (part of insert_sessions) to ensure tables are declared once data is in the database

Checklist:

• N This PR should be accompanied by a release: (yes/no/unsure)
• NA If release, I have updated the CITATION.cff
• N This PR makes edits to table definitions: (yes/no)
• NA If table edits, I have included an alter snippet for release notes.
• NA If this PR makes changes to position, I ran the relevant tests locally.
• I have updated the CHANGELOG.md with PR number and description.
• NA I have added/edited docs/notebooks to reflect the changes

[CBroz1]

Thanks, @samuelbray32!

I wondered if we should move this to part of the settings startup process, but that seems like a lot of cost to add to every session, rather than just protecting the 'populate all' startup process. I'm good with merging as-is

[CBroz1]

Suggested change

	"""Ensures all merge tables in the spyglass core package are declared.
	"""Ensures all merge tables in the spyglass core package are declared.