add support for custom offline atlases

[carlocastoldi]

With this small PR, i want to enable atlas developers to test their atlases using BrainGlobe's pipeline.
As of now, if the atlas is not official (i.e. present on the Gin repository), it updates the last_versions.conf file overriding any local changes that may have added a new custom local atlas.

This PR intends to read the atlases from both the cached file of last_versions.conf and custom_atlases.conf file.
custom_atlases.conf follows the same structure as last_versions.conf.

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

References

discussed over on Zulip.

How has this PR been tested?

python -c "import brainglobe_atlasapi as bg; print(bg.list_atlases.get_all_atlases_lastversions())"

with custom_atlases.conf file both existing and not.

Is this a breaking change?

no

Does this PR require an update to the documentation?

Perhaps?

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[carlocastoldi]

i don't think i introduced a bug that makes the test not pass, am i?

It's on atlasgen

[IgorTatarnikov]

Looks great! Thanks for adding this. Just a few semantic comments.

This change should be documented on the website but this can come in a subsequent PR, I'll make an issue to track this.