Create `bg-utils` package

[adamltyson]

We have a lot of duplicated code, and a lot of packages depend on imlib. It would be useful to develop bg-utils. This utility package would have:

• minimal dependencies
• core functionality relied upon by many brainglobe packages (e.g. Cell representations)

[adamltyson]

This now exists at https://github.com/brainglobe/brainglobe-utils

Need to move over a lot of imports from imio to brainglobe-utils

[adamltyson]

PRs raised to change the imports. Closing this issue now, but we should ensure that any common functionality is added to this repo.