Preserve map shape and dtype when a map is merged

[AhmetNSimsek]

Solves #302

After review, the same should be applied to neuroglancer.

[codecov-commenter]

Codecov Report

Merging #341 (3bf2547) into main (3426085) will increase coverage by 0.20%.
The diff coverage is 30.86%.

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@@            Coverage Diff             @@
##             main     #341      +/-   ##
==========================================
+ Coverage   35.87%   36.07%   +0.20%     
==========================================
  Files          60       60              
  Lines        5315     5316       +1     
==========================================
+ Hits         1907     1918      +11     
+ Misses       3408     3398      -10     

Impacted Files	Coverage Δ
siibra/__init__.py	66.66% <ø> (ø)
siibra/configuration/factory.py	22.47% <0.00%> (ø)
siibra/core/atlas.py	84.48% <ø> (ø)
...bra/features/connectivity/regional_connectivity.py	21.29% <0.00%> (+0.38%)	⬆️
...a/features/tabular/regional_timeseries_activity.py	28.75% <ø> (ø)
siibra/retrieval/requests.py	44.40% <0.00%> (ø)
siibra/volumes/neuroglancer.py	21.42% <0.00%> (ø)
siibra/volumes/nifti.py	18.91% <0.00%> (ø)
siibra/configuration/configuration.py	42.35% <11.11%> (-1.03%)	⬇️
siibra/volumes/parcellationmap.py	36.93% <33.33%> (-0.20%)	⬇️
... and 4 more

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