Cleanup of dlc video

[samuelbray32]

Description

Fixes #974
DLCPosVideo

• get epoch directly from key to avoid missing interval error
• default to opencv for video writing

Other

• minor bug fix from Allow dlc pipeline to run without prior position tracking #973

Checklist:

• This PR should be accompanied by a release: no
• N/A If release, I have updated the CITATION.cff
• This PR makes edits to table definitions: no
• N/A If table edits, I have included an alter snippet for release notes.
• I have updated the CHANGELOG.md with PR number and description.
• I have added/edited docs/notebooks to reflect the changes

[edeno]

Do you feel like there's a further issue here that there are no position intervals when DLC is used? I could imagine it messing up other aspects of spyglass.

[samuelbray32]

• I don't think that any of the spyglass package is dependent on that existing ( or at least LinearizedPosV1 and SortedSpikesDecoding don't).
• just making an interval wouldn't be enough to be able to restrict on it in the dlc tables (if that's the reason why desired). Would need to alter table(s) to add that as a secondary key

My current thought would be no, but I might be missing the general use case for it