Improve VideoInterface

[h-mayorquin]

Writing here a series of tasks to do for improving the movie interface:

• Add an example to the conversion gallery Add VideoInterface to conversion gallery #183
• Add timestamps stubbing. stub Movie interface timestamps #181
• Add another interface that converts a single movie file. The one we have now takes a list of movies and iterates over each one. Having a single movie interface would allow a simpler API for users, be easier to test, and allow us to do a good refactor of the current one
• Add a video example with variable rate to the gin-tests. Timestamp extraction and use with variable timestamps is not used..
• Add an example of the single movie interface to the conversion gallery. I think this case is way more common and does not require the users to wrap their file paths into a list.
• Separate the backend extraction from the conversion code. Right now the code for writing to nwb is tangled and mixed with the code for extracting properties from the videos with OpenCV. It is good practice to reduce the point of contact between functions (interfaces should touch each other at one place). This will simplify the code and if done well allow us to change the backend when needed without big headaches.
• Some videos have a starting time than OpenCV does not extract. We should look for an example of this, add it to gin tests and extract it automatically.

[pauladkisson]

Encountered some of these issues in the datta lab conversion: catalystneuro/datta-lab-to-nwb#56. VideoInterface could definitely use some love.