Add a save_any function to image_io

[K-Meech]

Image_io has a load_any function that recognises the image file extension and calls the appropriate loading function. It would be useful to add a corresponding save_any function, similar to here

[K-Meech]

We should also support saving a txt file with a tiff sequence as part of this. We already support loading such files via load_any and load_img_sequence

[adamltyson]

We should also support saving a txt file with a tiff sequence as part of this.

I don't think we have any need for this TBH. The image_io module is now just to support BrainGlobe tools, and none need to write to this format.

[alessandrofelder]

Do you think BrainGlobe tools will need to read this format @adamltyson ?

[adamltyson]

It was included for historical reasons to support brainreg & brainmapper. I think it's worth keeping as long as it isn't too much overhead.

[K-Meech]

I have a working version with tests updated for both save_any and txt file writing. I can move the txt file writing back to test_image_io if that's preferred though.