Add image IO loading code from cellfinder & create image IO submodule within IO

[adamltyson]

This PR does a few things:

• Moves data loading code from cellfinder to this package (see https://github.com/brainglobe/cellfinder/issues/405)
• Moves the image_io code into a submodule within IO (closes https://github.com/brainglobe/brainglobe-utils/issues/69)
• Updates some of existing image IO tests

Once merged, this should be released as 0.5.0 (to match the pinning in other packages)

Related PRs:

• https://github.com/brainglobe/brainglobe-utils/pull/73
• https://github.com/brainglobe/brainglobe.github.io/pull/184
• https://github.com/brainglobe/brainglobe-segmentation/pull/176
• https://github.com/brainglobe/brainglobe-atlasapi/pull/288
• https://github.com/brainglobe/brainreg/pull/199
• https://github.com/brainglobe/brainrender/pull/346

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.09302% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 89.76%. Comparing base (66e88a4) to head (a1a53b5).

Files	Patch %	Lines
brainglobe_utils/IO/image/load.py	67.56%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #73 +/- ## ========================================== - Coverage 90.38% 89.76% -0.62% ========================================== Files 35 35 Lines 1393 1427 +34 ========================================== + Hits 1259 1281 +22 - Misses 134 146 +12 ```

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