Grid Frame Offset option b

[darcymason]

I haven't observed an actual real-world problem with this, but noticed that GetDoseGrid, in using the GridFrameOffsetVector, does not check for the "option b" case defined in the DICOM standard, which could result in dose planes being offset from the correct location.

Unlikely, but possibly related to #160? Most likely the grid would be very far off if option b were used in the DICOM file.