BUG: fix some DICOMDiffusionPlugin issues
- 0.75 confidence is too high: non-DWI GE scans can have these tags set, no way to discriminate.
- set
loadable.selected
so that matches will actually be loadable (per @lassoan) - check the first three volumes because some (Siemens) scans won't have some tags for b0 volume.