Update imageio.imsave -> imageio.v3.imwrite

[mplough-kobold]

This PR updates documentation and code to use imageio's new v3.imwrite rather than the deprecated imsave. It uses the import, import imageio.v3 as iio, as recommended in imageio's narrative documentation.

Fixes #132.

[mplough-kobold]

Thanks @letmaik for the ping. I've now updated every use of imageio in the repo to use imageio.v3 -> iio. I also bumped the version of imageio required in dev-requirements.txt so we no longer need to do version checking. That eliminates the need for separate import imageio and import imageio.v3 as iio lines.