move niftyreg to separate optional package

- [x] `nimpa[niftyreg]` #25 
  - [ ] make `nipet` depend on `nimpa[niftyreg]` (for now)
- [x] move to `niftyreg` to separate package (similar to `spm12`) https://github.com/KCL-BMEIS/niftyreg/pull/80
  - [x] add python wrapper function (rather than directly calling `subprocess`)