Proposed changes

Currently ukat only supports ascending slice acquisition orders i.e. ukat assumes the zeroth slice was acquired first, next slice tss later, the next slice 2 x tss later etc. This isn't always the case. In descending acquisitions, the -1th slice is acquired first, the -2th slice tss later etc and in centric acquisitions the slice acquisition order is [4, 2, 0, 1, 3] for a five slice acquisition. ukat should support these options and, ideally, custom acquisition orders.

Checklists

• I have read and followed the CONTRIBUTING document
• This pull request is from and to the dev branch
• I have added tests that demonstrate the feature/fix works
• I have added necessary documentation (if appropriate)
• I have updated documentation which becomes obsolete after my changes (if appropriate)
• I have added/updated a notebook to demonstrate the changes (if appropriate)
• Files added follow the repository structure (if appropriate)