Use arbitrary root during init

[IgorTatarnikov]

Before submitting a pull request (PR), please read the contributing guide.

Please fill out as much of this template as you can, but if you have any problems or questions, just leave a comment and we will help out :)

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
Currently, brainrender hardcodes a structure named "root" to use as the root mesh. This may lead to errors if an atlas exists but doesn't call the root node "root".

What does this PR do?
Generalise the current code by using the id of the root node of the atlas hierarchy to fetch an arbitrarily named root mesh to initialise the scene.

How has this PR been tested?

All tests pass, and examples checked by visual inspection.

Is this a breaking change?

No

Does this PR require an update to the documentation?

No

Checklist:

• The code has been tested locally
• The code has been formatted with pre-commit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (6476c43) to head (6dd0441).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #474   +/-   ##
=======================================
  Coverage   88.54%   88.55%           
=======================================
  Files          27       27           
  Lines        1318     1319    +1     
=======================================
+ Hits         1167     1168    +1     
  Misses        151      151           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.