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 #327. 3) Add specific unit-testing and re-enable round-trip tests.
DWIobject by spinningto_niftioff from the type.to_niftiis 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.