Remove all references to the transformation to the BrainGlobe orientation

Removed all internal logic and references related to transforming atlas data to the BrainGlobe orientation, requiring input data to be pre-oriented.

What was done

The transformation_mat parameter and the trasform_to_bg key were removed from the generate_metadata_dict function in metadata_utils.py.
All orientation transformation logic was removed from the wrapup_atlas_from_data function in wrapup.py. This included removing calls to map_stack_to and map_points_to, and the calculation/storage of a transformation matrix.
An assertion was added to wrapup_atlas_from_data to enforce that input stacks and meshes are already oriented to "asr".
The orientation field was removed from the metadata template in descriptors.py.
Removed the import of brainglobe_space and the instantiation of AnatomicalSpace objects, as they are no longer needed for internal transformations.

Atlas generation now strictly expects input data (stacks and meshes) to be in the "asr" (Anterior-Superior-Right) BrainGlobe orientation, making the API simpler but requiring users to pre-orient their data.
This is a breaking change for atlas generators that previously relied on brainglobe-atlasapi to perform orientation transformations.
Improved clarity in atlas metadata by removing redundant transformation information.
Minor performance gains during atlas generation due to the removal of transformation computations.

Files touched include brainglobe_atlasapi/atlas_generation/metadata_utils.py, brainglobe_atlasapi/atlas_generation/wrapup.py, and brainglobe_atlasapi/descriptors.py.
The generate_metadata_dict function no longer accepts a transformation_mat parameter or adds trasform_to_bg to the metadata.
The wrapup_atlas_from_data function was refactored to remove all brainglobe_space dependencies and related orientation logic.
The METADATA_TEMPLATE in descriptors.py was updated to no longer include an "orientation" field.
The change shifts the responsibility of data orientation to the data provider rather than the atlas packaging utility.