add MAGNITUDE to ImageType for GE data

[bpinsard]

That change is necessary for heudiconv logic in heudiconv.convert.update_complex_name triggering RuntimeError("Data type could not be inferred from the metadata.").

https://github.com/nipy/heudiconv/blob/68231ef2dc9bfc603c2f925c3ec00c98dd6681b4/heudiconv/convert.py#L334-L340

Otherwise, it would require heudiconv to assume MAGNITUDE by default, while we reliably have that information from the dicoms at the conversion stage.