Add save_any function to image_io

[K-Meech]

Closes https://github.com/brainglobe/brainglobe-utils/issues/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