Calling Save Images from Python Interface

[KevinJ90825]

Description of Problem & Solution
I would like to be able to generate images for split scenes with the Python API, in the same way you can generate them with the CLI save-images command

Proposed Implementation:
The addition of an API method in the video-splitting module, ideally, or wherever else might be deemed appropriate.

[Breakthrough]

Implemented global function scenedetect.scene_manager.generate_images in v0.5.2-dev, and still TODO is to add an actual save_images method to the SceneManager which will invoke the generate_images function.

[Breakthrough]

This function will need to be modified either as part of #123 / #125 or another changeset afterwards. If #123 / #125 have to be deferred to v0.6 because they require breaking API changes, this will be implemented first in v0.5.2 (as it is a non-breaking API change).

[Breakthrough]

After #129 is merged, work on this will resume, which also needs to write unit tests for the new global function.

[Breakthrough]

This is now available in the v0.5.5 branch. It can be called via scenedetect.scene_manager.save_images now. Targeting v0.5.5 release now.

TODO:

• finish documentation, add to manual [done (link)]
• add unit tests [done]
• update changelog [done]

[Breakthrough]

This has been included in the v0.5.5 release. Documentation for the save_images() function can be found here (link).