Introduce merging Neuroglancer fragments

[AhmetNSimsek]

When a map with fragmented Neuroglancer volumes is requested without specifying a fragment, the fetch fails as merging of fragments for Neuroglancer volumes was not implemented (#551). Test with:

from nilearn import plotting
import siibra
mp = siibra.get_map('cortical layers', space='bigbrain')
res = 0.64
img = mp.fetch(resolution_mm=res)
bigbrain = siibra.get_template('bigbrain').fetch(resolution_mm=res)
plotting.view_img(img, bg_img=bigbrain, cmap=mp.get_colormap(), symmetric_cmap=False)

Also, fixes voxel dimension calculation during mergin fragments of nifti volumes. (This was not a problem most of the time since the voxel size of most fragmented nifti is 1 mm)

[codecov-commenter]

Codecov Report

Attention: 1315 lines in your changes are missing coverage. Please review.

Comparison is base (5a0e6b3) 36.81% compared to head (9be5f5d) 53.28%. Report is 387 commits behind head on main.

Files	Patch %	Lines
siibra/features/feature.py	48.55%	125 Missing :warning:
siibra/volumes/volume.py	44.85%	118 Missing :warning:
siibra/core/region.py	56.75%	112 Missing :warning:
siibra/volumes/parcellationmap.py	18.97%	111 Missing :warning:
siibra/explorer/url.py	0.00%	107 Missing :warning:
...bra/features/connectivity/regional_connectivity.py	33.33%	56 Missing :warning:
siibra/retrieval/datasets.py	49.53%	54 Missing :warning:
siibra/explorer/util.py	0.00%	51 Missing :warning:
...a/features/tabular/receptor_density_fingerprint.py	4.54%	42 Missing :warning:
...a/features/tabular/regional_timeseries_activity.py	39.13%	42 Missing :warning:
... and 36 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #561 +/- ## =========================================== + Coverage 36.81% 53.28% +16.46% =========================================== Files 61 69 +8 Lines 5421 6645 +1224 =========================================== + Hits 1996 3541 +1545 + Misses 3425 3104 -321 ```

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