0.5 voxel affine shift of neuroglrancer/precomputed sources

[AhmetNSimsek]

siibra-python (v1.0a19 an prior) fetches the affine neuroglrancer/precomputed images from transform.json files stored along with the images (baseurl/transform.json). However, these transform files are only meant for neuroglancer viewer to begin with. Therefore, it is NeuroglrancerProvider's job to correctly fetch the affine correctly from the source. Will be remedied with 1.0a20.

Before closing update:

• [ ] bounding boxes of ng providers in configs
• [ ] update tests concerning BigBrainIntesnsityProfiles

[AhmetNSimsek]

@xgui3783 This concerns the explorer as well since centroid calculations can be shifted if there is only a neuroglancer/precomputed source. I will also update v0.4EOL.