Add save_any function to image_io

[K-Meech]

Closes #50

This PR:

• Moves save_any from test_image_io into the image_io module (removing support for txt files)
• Changes text file writing into a fixture inside test_image_io
• Moves to using load_any() wherever possible in test_image_io
• Removes test_load_img_sequence_from_txt and test_nrrd_io, that are already covered by test_load_any_txt and test_save_and_load_any
• Adds in a few extra assertions for scaling / correct file number in test_image_io

[alessandrofelder]

Thanks @K-Meech - one tiny comment.
Looking good 🎉 feel free to merge after considering it!

[alessandrofelder]

Should we generalise this to something like

Suggested change

	random.Random(4).shuffle(tiff_paths)
	random.Random(len(tiff_paths)).shuffle(tiff_paths)

or specify in the docstring that the function only shuffles the first four paths?

[K-Meech]

@alessandrofelder this is actually the random seed, it just happened to be equal to the number of paths in this case! I set this to ensure we're getting the same result each time, and avoid the cases where it might shuffle the paths back into the correct order accidentally.

[alessandrofelder]

Ha sorry! All good then, thanks!