ENH: New philips data has new tags

In newer philips data the gradient field tags are not 12b0,12b1,12b2 instead of 10b0,10b1,10b2
BUG: Read gradient directions if they exists

The logic would avoid reading gradient directions
if the DiffusionDirection was "NONE", but some newer
dicom files do not populate the necessary feild as "DIFFUSION",
and the result is that all gradients ended up being interpreted
as B0's.

This re-organizes to attempt to get gradient directions if
they are available, and prefering the new Supplement49 tags
if they exists.
COMP: simplified error handling.

No need for bool variables set in exceptions to determine if alternate tags
need checking.
Merge pull request #72 from BRAINSia/FixForNewPhilipsDWI

Fix for new philips dwi