WIP: Fix off-by-one errors in conform.py if flipping orientation

[dkuegler]

This PR is a work in progress PR to address the issue of MGH Headers using Pxyz_c (which is not the center of the image, but offset by half a voxel).
This is also a bug in mri_convert as documented in freesurfer/freesurfer#1358

Todo:

• test whether the current solution fixes all issues with re-orientation and RAS consistency of images

[Copilot]

Pull request overview

This work-in-progress PR addresses off-by-one errors in MGH header handling related to the Pxyz_c calculation, which represents the center of the image but is offset by half a voxel. The fix introduces two feature flags to optionally correct the affine calculation bug in nibabel's MGHHeader.get_affine() method.

Changes:

• Introduces two feature flags (FIX_MGH_AFFINE_CALCULATION and FIX_CENTER_NOT_CENTER) to control the fix behavior
• Adds helper functions get_affine_from_mgh() and get_affine_from_any() to provide corrected affine calculations
• Modifies offset calculation in map_image() to account for off-by-one errors when flipping orientation
• Updates center calculation logic in prepare_mgh_header() with conditional fix based on feature flags

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