Fix FFMPEG issue

[jemrobinson]

• use imageio-ffmpeg to install FFMPEG
• set the appropriate matplotlib configuration to allow this to be used by default

Closes #215

P.S. I haven't tested this - if someone who's experiencing the problem can do so, that would be great.

[github-actions]

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
icenet_mp
plugins.py
icenet_mp/visualisations
__init__.py
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[louisavz]

I can give this a go and test it. What does good look like? The issue said "Revert ffmpeg dependency", we should still see the animation but now rely on ImageIO's library?

[jemrobinson]

I think the best way to test is to do a minimal train run (e.g. with a simple model and 1 or 2 epochs) and then run evaluate on the checkpoint from that run. Evaluate is the step that runs the video generation.

• It should not print the line Video plotting skipped: Video encoding failed: [Errno 2] No such file or directory: 'ffmpeg'.
• It should produce the .mp4 video which gets uploaded to W&B.

[jemrobinson]

@alan-turing-institute/sea-ice-prediction-developers: this is working for me on Isambard and it's been waiting for review for 2 weeks. If there are no objections I will merge it today.

[IFenton]

@jemrobinson Do you want me to have a quick look at it first?

[jemrobinson]

@IFenton if you have time but I'd rather have your focus on getting #216 merged. This PR fixes (or attempts to fix) something that's currently broken but doesn't seem to be problematic.

[IFenton]

LGTM. It produces the videos on Baskerville, which I think is where the problem was before