Merge table efficiency

[samuelbray32]

Description

Changes to help speed up fetching nwb data through merge tables

• allow Merge.fetch_nwb to work with a restriction containing multiple entries and sources
• pre-check dependencies loaded to in _ensure_dependencies_loaded
  • Avoid the overhead of the load call
• avoid redundant calls to merge_get_parent in merge_restrict_class
• refactor to single fetch_nwb call in SortedSpikesGroup.fetch_spike_data
  • avoids repeatedly running the merge table restriction process to find source tables

Checklist:

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

[CBroz1]

Super minor request to rename a variable - otherwise good to go