Create animations in horizontal view

[Andreas Hilboll]

It would be useful to create / view animated forecast steps in the horizontal view.

For example, for a selected init_time and valid_time, create an animation with the following n valid_times.

[Reimar Bauer]

May be we could do a webm file, and offer it for download. Needs more thinking.

[Reimar Bauer]

hmm, without any modification a user could do that by peek or similar tools. https://github.com/phw/peek

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[Reimar Bauer]

Duplicate of #514.