Revise and exercise `DWI.to_nifti`

[oesteban]

1. We probably want to lighten up the DWI object by spinning to_nifti off from the type.
2. We need to investigate why to_nifti is so inefficient (it takes forever even when the motion matrices are identity/None). We (I) disabled testing in REF: Refactor gradient data checks in DWI data class #327.
3. Add specific unit-testing and re-enable round-trip tests.