FCP_XML adapter: for media start time timecode attribute, ignore if timecode rate not in prespecified known timecode list

## Feature Request

Modification to existing behaviour

## Description

Currently, OpenTimelineIO throws an exception if the framerate of a video is not one a predefined set of allowed framerates: https://github.com/AcademySoftwareFoundation/OpenTimelineIO/blob/e2a8594a9a6b992ee1b7bb776b1281da8dbc090d/src/opentime/rationalTime.cpp#L42-L57 Unfortunately, QuickTime screen recording records videos at arbitrary framerates, depending on current system load.

I tried just removing the framerate check (https://github.com/pokey/OpenTimelineIO/commit/844d50b6ff00556be08c8675b1b5c64e609060d8), and everything seems to work fine.

Is there some reason that check needs to be there?

## Context

N/A

Start with the linked src/opentime/rationalTime.cpp framerate validation and trace how the FCP_XML adapter handles the media-start timecode attribute. Check whether existing tests cover unsupported timecode rates; done means the adapter ignores rates outside the known list without breaking handling of known rates.

Written by the indexing model from the issue text.