Maint: `volume.fetch`

[AhmetNSimsek]

During the last refactoring, this method has been changed a few times, leaving some artifacts and small issue to resolve. This PR addresses these.

• Clean up the code
• Repeat only in case of too many requests
• Check if "resolution_mm" supplied without format and set to NG (this behaviour already existent in parcellationmap.Map.fetch() but this is going to be removed as it is not it's job.) (Alternatively, the fetch can fail at, say NiftiProvider.fetch() but it is not user's job to know which formats are shipped with different resolutions. And since we decided against resampling nifti images if resolution_mm is supplied, this seems to be the best solution at the moment.)
• formats property should only list actual formats otherwise recursion occurs.

[codecov-commenter]

Codecov Report

Attention: 1305 lines in your changes are missing coverage. Please review.

Comparison is base (5a0e6b3) 36.81% compared to head (3ddcb1e) 53.29%. Report is 387 commits behind head on main.

Files	Patch %	Lines
siibra/features/feature.py	48.55%	125 Missing :warning:
siibra/volumes/volume.py	43.89%	124 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 #562 +/- ## =========================================== + Coverage 36.81% 53.29% +16.47% =========================================== Files 61 69 +8 Lines 5421 6638 +1217 =========================================== + Hits 1996 3538 +1542 + Misses 3425 3100 -325 ```

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