Add extra unit test to ConvertToDiffractionMDWorkspace3Test to cover
ISIS use case for ConvertToDiffractionMDWorkspace where initial workspace
has units of dSpacing.
Also fix bug in ConvertToMDMinMaxLocal (which is called as a child
algorithm from ConvertToDiffractionMDWorkspace) where there's a mix up
between the workspace index and the detector index used to look up stuff
from the table workspace created by PreprocessDetectorsToMD