Proper handling of VOI specifications when fetching across neuroglancer fragment volumes

[dickscheid]

When fetching from a multi-fragment neuroglancer provider with a voi, the current implementation returned a full-size but almost empty volume. This PR tries to fix the problem, and had to address a couple of downstream issues with neuroglancer fetching.

It needs testing and independent review...

The design of neuroglancer volumes, scales, providers etc. appears overcomplicated and makes it difficult to solve this elegantly, but solving the higher level design will be part of siibra-python v2.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 19.51220% with 66 lines in your changes missing coverage. Please review.

Project coverage is 47.48%. Comparing base (6ada413) to head (4bb16b4). Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
siibra/volumes/volume.py	20.00%	32 Missing :warning:
siibra/volumes/providers/neuroglancer.py	28.57%	20 Missing :warning:
siibra/volumes/parcellationmap.py	0.00%	10 Missing :warning:
siibra/core/region.py	0.00%	3 Missing :warning:
siibra/locations/experimental.py	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #656      +/-   ##
==========================================
- Coverage   47.49%   47.48%   -0.02%     
==========================================
  Files          71       71              
  Lines        7430     7432       +2     
==========================================
  Hits         3529     3529              
- Misses       3901     3903       +2     

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